First let me start by saying that I am a HUGE fan of the trilogy. I own the extended version, and I have watched it a half dozen times. I think it's one of the best movie series ever made.
That being said, allow me to write something about the logical problems in the story of the movies. I am going to write about the movies rather than the books, as the last time I read the books was something like 15 years ago and I remember next to nothing about them. Many of these problems may be identical to what is in the books, but some might be specific to the movies themselves.
Thus everything I write is from the pure perspective of what the movies depict. The books go into much greater detail on many things, but I am not writing from that perspective.
The movies don't explain too profoundly what the Rings of Power are, except for the speech at the beginning of the first film. If I had to deduce what they were trying to say, it would be something like this:
A very long time before the movie settings, Sauron, some kind of extremely powerful being, made some Rings of Power (what those powers are is never specified) and probably acting as a benevolent being gave these rings to the most prominent leaders of elves, dwarves and men, which were the three dominant species of mortal beings on Middle Earth at the time.
However, Sauron was not benevolent at all, but completely evil, and his plan was to dominate the three species by controlling their leaders through these rings. To do this he created a "master ring", "The One Ring" which he could use to dominate the others. The movies don't explain at all why it was necessary to create such a ring and why Sauron couldn't simply dominate the other rings directly, but whatever.
The movies also don't explain at all what happened then. Were these dominant species dominated or not? Apparently the plan failed at least at some point because none of the three species were under the dominion of Sauron. On the contrary, at some point men and elves raised against Sauron and defeated him and his forces in battle.
So it seems that Sauron's plan was an utter failure. And in the process he made himself extremely vulnerable and mortal, but I'll dig deeper into that detail later. One would think that such a powerful being would be smart enough to not to fail in such a catastrophical way, but whatever.
The movies reveal what happened to the nine kings of men who were given nine of the rings: They were completely possessed by evil and converted into immortal demonic creatures. For some reason, at least at some point in history, they were still not able to rule the species of men under Sauron's control.
The movies make absolutely no mention about what happened to the dwarf leaders who were given the rings.
The movies do reveal what happened to at least one elf who was given a ring: She still has it, even after thousands of years, and probably uses it.
The movies do not dig any deeper into this. It doesn't seem to make too much sense. Surely this elf knows that the ring is evil and its only purpose is for Sauron to dominate it using the One Ring. Yet she still has it and she uses it. The only possible explanation is that the elves are too strong to being dominated by these rings, and that they can use them freely as they wish, without any danger of being dominated by Sauron.
If this is so, then Sauron's plan failed even more catastrophically as he didn't foresee that elves could not be dominated this way. The movies give the impression of Sauron being an extremely powerful being, but it seems that he was not powerful enough to know that elves could not be dominated this way, and basically gave them three powerful magical items for free. Why wouldn't he know this?
(One could argue that he did know this, but he gave them the rings anyways in order to lure the other species to join in, that it was all part of the master plan, and an acceptable sacrifice. Whatever.)
So far the problems have been only minor. However, the One Ring just doesn't make any sense at all.
Sauron, an immortal almost-all-powerful being, puts most of his life force into a small magical ring in order to control the other rings. By doing this he basically makes himself extremely vulnerable and mortal. As seen in the beginning of the first movie, it's enough to cut his ring finger to kill him almost completely.
This makes absolutely no sense. Why would such a powerful being make himself so vulnerable and mortal, no matter what kind of world domination plans he had? Surely such a powerful being did know how vulnerable he would become if he makes such a ring? Wasn't it utterly stupid to do so, no matter how ingenuous the world-domination plan (which then even failed monumentally)?
Also, Sauron is able to sense the location of the One Ring when someone wears it, but not otherwise. Why wouldn't he make the ring so that he could always detect its presence?
And why was it enough to simply remove the ring from Sauron's body in order to kill him? How does that make any sense? Not only did the ring make Sauron very vulnerable, killing him was laughably easy: Just remove the ring and you have killed Sauron, one of the most powerful beings in this world.
Not only that, but destroying the ring itself would completely annihilate Sauron. Surely Sauron did know this as well. Why make himself vulnerable and mortal, and completely killable by simply destroying the ring? Even if the ring could only be destroyed in Mount Doom, why would he risk even that?
And related to that: Given that Sauron knows that the ring can be destroyed in Mount Doom, why was the entrance completely unguarded? Sure, the risk of anyone walking through Mordor with the ring and entering Mount Doom to destroy it was next to none, but why take even that little risk?
Why not seal the entrance completely so that nobody can enter? Or if the entrance must be kept accessible for whatever reason, surely he could spare a couple of dozens of orcs to guard the entrance? Or at least put a heavy locked door or whatever. With all the magic and wizards at his disposal, maybe some kind of magical barrier? Anything but leaving it completely unguarded and freely accessible, which is just plain stupid and doesn't make the least amount of sense.
All this seems to imply that Sauron was completely stupid, which doesn't make too much sense for such a powerful immortal being. Seemingly all of his minions (including Saruman) were also completely stupid because nobody seems to have suggested Sauron any of this either. Surely at least Saruman would have suggested to Sauron to seal or put guards at the entrance of Mount Doom?
The only power of the One Ring shown in the movies is making the wearer invisible. However, the movies hint at the ring having many other powers as well, but they are never specified. Some dialogue related to this doesn't make too much sense. For example Boromir says:
It is a gift. A gift to the foes of Mordor. Why not use this ring? Long has my father, the Steward of Gondor, kept the forces of Mordor at bay. By the blood of our people are your lands kept safe. Give Gondor the weapon of the enemy. Let us use it against him.
Exactly how was Boromir suggesting to use the ring against the forces of Mordor? Putting the ring on would only alert its presence to Sauron, and the invisibility does not help against the Ring Wraiths, so the ring would be basically useless for fighting against Mordor this way.
Ok, maybe Boromir did not know this. However, nobody else present at the time brought up the issue. (For example Gandalf did know this perfectly, as established earlier in the movie.)
Even with invisibility there's just so much you can do against an army of hundreds of thousands of orcs and other creatures. The invisibility of one person just doesn't sound like such a big power against an entire army. However, Boromir acts like he knows exactly how powerful the ring is. He (as well as others in other situations) makes it sound like it was some kind of superweapon which could be used to annihilate entire armies.
How exactly did Boromir know of the real powers of the ring, and why aren't they specified in the movies? Ok, this isn't a very big deal storywise, but it's a bit annoying because the viewer is kept in the dark and never told exactly how powerful the ring really is.
The prologue establishes that the existence of the One Ring was forgotten as millenia passed. Also, most importantly, the beginning of the first movie clearly establishes that Gandalf doesn't know too much about the One Ring. Even if he knows about its existence, he doesn't know about the details. He travels to Minas Tirith and searches old chronicles until he finds the story of the ring. All this is clearly new information to him.
However, later in the movie suddenly everyone seems to know about the ring. In a scene in the third film (only in the extended version), in a flashback which happens before the Council of Elrond (which happened in the first movie) Denethor, the Steward of Gondor, talks to Boromir (his son) and mentions about rumors of the One Ring having been found.
So the One Ring started by being a completely forgotten artifact which even the mighty Gandalf didn't know too much about, and even when he did find about it, he made sure its existence would be kept secret. Later it seems that everyone knows about the ring, and rumors about it having been found are widespread. Everyone talks about "the ring of power" as if it was common knowledge what it is.
While it might be remotely explainable how the knowledge got widespread after Elrond summoned the council at Rivendell (although the movie hints that at no point the reason for the council was specified to anyone), it still feels very inconsistent.
The Eagles don't make too much sense storywise. They help Gandalf out of the tower of Orthanc when Gandalf asks for their help, they help in the final battle at the gates of Mordor, they help taking Frodo and Sam out of Mordor after Mount Doom erupts.
However, why couldn't they help in other situations as well? Would it have been such a huge burden to carry the ring bearer and his party members to Mordor? Or if they didn't want to fly to Mordor, at least closer to it, so that they could have saved miles and miles of walking in dangerous enemy-infested territory?
In the first movie Gandalf definitely wouldn't want to go through the mines of Moria, so he wants to go over the mountains. That plan doesn't work, so they are forced to go through the mines. Gandalf keeps oddly quiet about the idea of asking the Eagles to transport them over the mountains. What would have been the problem?
I think the entire story would have been a bit more logical without the Eagles. They are too powerful benevolent flying beings, and thus they introduce too many unanswered questions about why they sometimes help while most of the time they are not even mentioned and nowhere to be seen, even though they could be of great help.
Gandalf's magical powers seem very erratic at best. And they also seem to depend on who he is fighting against.
When fighting against a mighty Balrog, an extremely powerful demon from ancient times, he can create a forcefield around himself which protects him from the Balrog's attacks, he can summon lightning, and overall he can fight as an equal to the mighty demon beast.
When fighting against the Ring Wraith, he can spawn a ray of energy from his staff which draws them away. When fighting against another equally powerful wizard (Saruman) he can use powers akin to telekinesis.
However, none of these powers are used or present when he is fighting against orcs. He just uses his staff as a bludgeon, but that's about it. No protective forcefields, no telekinesis, no summoning lightning... nothing.
It seems that Gandalf's powers fluctuate so that they are always in par with the powers of his foe. If his foe has no magical powers, neither does he.
(One could perhaps come up with a handwave for this by inventing, for example, some "code of honor" or other type of guideline that Gandalf, who is clearly not a man but some kind of demigod or similar otherwordly creature, must obey. However, this would feel a really artificial restriction that serves no useful purpose and only defeats the whole idea of Gandalf being there to aid the inhabitants of the world against the forces of evil.)
It's funny how the movies succeed in depicting the elves as powerful, benevolent and very noble people, who are cultural and intellectual peaks of this world, yet when you stop and think about their actions in the movies, they actually turn out to be quite different from that.
The prologue establishes that in the distant past elves were allies of men and fought alongside them to defeat Sauron, at great cost.
In the timeline of the movies, however, they are outright cowards who couldn't care less what happens to the world and its inhabitants. When news about the return of Sauron (who was still quite weak at that point by all accounts, only a shadow of his former self) and the gathering of Sauron's army reached the elves, what do they do?
They flee. They abandon the world and all of its inhabitants to their fates, and couldn't care less what happens to them. Where is their honor, duty and friendship towards their human allies?
(A group of elves go to aid Rohan in their battle against Saruman's army, and they specifically mention their past alliance as a reason. This actually makes sense, but it's an addition in the movie that was not in the original book. Ironically, many purists detest this addition, as if it somehow made the movie worse. "Whaaat? Elves showing some integrity and honor, and keeping up their alliance with men? No no no, that's wrong! They should be depicted as the cowards they are!")
Both Helm's Deep and Minas Tirith seem rather poorly designed to withstand enemy attacks, especially the former, given that it's a stronghold specifically designed to keep attacking forces at bay for as long as possible (rather than being a place of living.)
There's a very simple but effective invention that would have made both places a lot, lot more difficult to invade. It's called a moat.
In neither case was there any discernible logical reason why a moat couldn't have been built outside the walls, especially with Helm's Deep (since the wall was just some 50 meters or such long.) A deep and wide moat would have made invasion significantly harder, even if it had been dry (especially if you put spikes and other deadly things on the bottom.) In fact, it's shown that there's water flowing out of the deep, so it would have filled the moat nicely.
Also, when the invading forces succeed in breaching the main gates of Helm's Deep, then they start to further reinforce them from the inside. Why? What exactly stopped them from putting as much reinforcement to the gates when they still had time? The gate was the weakest point of the entire structure, so it makes no sense to not to reinforce it as much as possible.
Speaking of which, there's another invention that would have made the invasion more difficult: A drawbridge in front of the main gate. It's difficult to ram the gate when there's a 10 meters wide deep chasm in front of it. This would have also helped at Minas Tirith (alongside with the moat.)
And that's just a couple of the most obvious defense mechanisms that could have made both places a lot more secure.
And related to this same issue...
Or the complete lack of.
It's just amazing how the armies, especially those of Rohan and Gondor, seem to completely lack even the most basic concepts of warfare.
Consider, for instance, the grand battle at Minas Tirith, and what the army of Rohan did when it arrived at the battle. It announced itself with horns and standing very visibly on the horizon, completely blewing up any chance of a surprise attack and giving the orc army plenty of time to prepare. Then they just rode head-on onto the enemy with no organization or tactics of any kind. Just a mindless brawl.
Even the stupidest of leaders would have thought of trying to take even some advantage of their special situation (in that the orc army was not expecting the Rohan army to arrive at that moment.) Take them by surprise, flank them, approach them from a direction that they least expect, attack them from behind and several other directions, cause confusion and give them little time to regroup and organize a defensive position. Anything but just a blind frontal assault into a mindless brawl, after giving the enemy plentiful warning of their arrival.
Sure, it was a suicide attack to begin with, but at least they would have made significantly more damage to the orc army, and increased the chances of Gondor to do something even if so slightly.
It's established that the ring can affect at some level people around it (even if they are not touching or see the ring) and this capability may get stronger as the movies progress, as Sauron's strength increases. (It's not clear if the ring has a kind of will of its own, even though it's stated that the ring "wants" to get back to its master, and therefore is getting more desperate as it's getting closer to Mordor, or whether it's Sauron's increasing power that's indirectly fueling the ring, but it doesn't really matter all that much.)
When Frodo and Sam are captured by the soldiers of Gondor, led by Faramir, the ring sees its opportunity. Men are much, much more easily corruptible than hobbits, and if a man like Faramir were to just take the ring, its goal (ie. returning to Sauron) would be all but certain to be achieved. Therefore it tries its best to corrupt and tempt Faramir to either take the ring or bring it to his father. And it succeeds in this. Faramir is manipulated by the ring to take it to his father.
I suppose this makes sense. What makes a lot less sense is why Faramir didn't just take the ring. Why bring the hobbits along, and let them keep the ring? (Although one could argue that at some level Faramir was actually afraid of touching the ring, given that he had probably heard stories of how it corrupted Isildur.)
Faramir taking Frodo and Sam to the city of Osgiliath is not in the books, and is an addition in the movies. Nevertheless, that's not the worst problem with this. No, the worst problem is why Faramir changes his mind. It doesn't make much sense.
All the way to Osgiliath, and in there, Faramir is not persuaded by Frodo's or Sam's pleads. He is determined to bring the ring to his father.
So what did persuade Faramir? He saw Frodo offering the ring to a Ring Wraith, and Sam stopping him. Right after that Faramir let them go, even forfeiting his own life in the process. What? Why? That doesn't make any kind of sense. If anything, that should have convinced even further that it's unsafe to let the hobbit go with the ring.
Which, by the way, brings us to the next point.
It's actually annoyingly nonsensical how easily the ring wraiths give up trying to take the ring, even after directly seeing that it's there, and it basically being within their grasp. It happens several times throughout the movies.
This makes very little sense. It would make a lot more sense that once they see the ring, they would fight like there's no tomorrow to get it. No matter if there's an army of a hundred thousand soldiers, they would fight and fight to get the ring. After all, they cannot die, so it's not like they have anything to lose. By giving up so easily, they have everything to lose.
Also, in the extended version, the Witch King is about to kill Gandalf and... just gives up as soon as he hears the horn of Rohan. For no sensible or rational reason whatsoever. Why would he care if the Rohirrim have arrived? Let the hundreds of thousands of orcs handle them. And even if for some strange reason he just had to deal with them, it's not like he was in any kind of hurry to do that at that precise moment. Surely he could spare a few minutes, or even tens of minutes, to fight with Gandalf, who was a much higher priority target in that moment.
(Ironically, when he's about to kill the king of Rohan, he does not give up at the first sign of trouble, and dies because of that. The one instance where giving up would have been more beneficial than pressing on, and now he chooses to do the latter. Plot-driven stupidity to the max.)
When Minas Tirith signals Rohan to come to aid them, they (or rather, Gandalf) light beacons, which are basically bonfires at mountaintops at sight of each other.
Which raises tons and tons of questions. Most of these beacons are shown to be at the top of snowy mountain ranges with no villages, roads or any kind of civilization in sight, other than a really small cottage near each beacon. Do the people watching over the beacons live there 24/7, isolated from the rest of the world? How do they get food and water? (Most of those mountaintops are completely barren and covered in snow.) How exactly do they live there? Are they watching the neighboring beacons 24/7 just in case they are lit? How did they get there in the first place? (As said, there are no roads or anything in sight, and the terrain is basically impassable.)
(To be continued...)