You know, a lot of the decisions Tirian and Jewel made were ones where it's impossible to say what was the right thing to do. If they hadn't immediately gone to Lantern Waste without backup, the criticism would be that they weren't acting quickly enough to save any dryads. If they hadn't turned themselves over to the Calormenes, the criticism would be that they weren't taking responsibility for killing those unarmed men without a warning of warning. Sheesh, you could even argue that if they hadn't done that dreadful deed, that would have been seen as a lack of proper concern for the horse those Calormenes were abusing. It feels like there weren't a lot of obvious options.

I agree with @col-klink, there weren't that many, if any, options that wouldn't have had negative repercussions. I have never considered that Tirian was a neglectful/irresponsible king (not saying he wasn't, it just never crossed my mind before) but I have a feeling the state of affairs in Narnia in The Last Battle were a long time coming, as are most world events with widespread consequences, and while Tirian may have contributed to the downfall of Narnia he was by no means solely responsible.

Posted by: @courtenay

Yet this fatalistic sort of doom mentality hangs over them and spurs their actions

But this mentality isn't restricted to the King and the Unicorn - it seems to be the mentality of all the Narnians at that time. They are quite willing to believe the Ape, even though they ought to have recognised the truth, just because of everything they should have known about Aslan and about his history of protecting Narnia.

I think their mentality, more than that of the King, is why I agree with you that this book "is SO darn depressing, most of the way through".

@col-klink and @narnian-in-the-north, I agree there don't seem to have been many good options available to Tirian and it's hard to say whether he could have done anything that wouldn't have had negative consequences. And there does also seem to be a sense that the state of affairs in Narnia has built up gradually, and as @varna says too, that doom-and-gloom mentality seems to have overtaken the other Narnians, most of whom seem to believe Shift and the Calormenes, however despairing they may be about it.

And that's what keeps bugging me about this book, now that I'm revisiting it for the first time in many years. How did so many Narnians get so easily fooled about the true nature of Aslan? Even if the last time he visited Narnia personally was during the events of The Silver Chair, that's only 199 years before the events of The Last Battle (going by Lewis's later timeline, which I now see has yet another discrepancy... Tirian tells Jill and Eustace in LB that Rilian "has been dead over 200 years", but the timeline gives Narnian year 2356 as the year of the events in SC and 2555 as the year of the events of LB, which is under 200 years). That's shorter than the lifespans of at least some Narnian creatures, as we can infer from what Roonwit says. And even in the 1,303 years between the events of LWW and PC, not everyone forgets Aslan, even over the 305 of those years that Narnia is under Telmarine rule and the Talking Beasts and other native Narnian peoples are in hiding. Trufflehunter tells us repeatedly that "we don't change, we Beasts... We don't forget."

Yet virtually everyone in Narnia, by Tirian's time, is duped by an "Aslan" who doesn't show himself to them except in very dim light, doesn't speak to them, uses an Ape as a mouthpiece, and apparently orders Dryads to be slaughtered and Talking Beasts to be enslaved and Calormenes — Narnia's ancient enemies, who worship a god utterly unlike Aslan and who have wanted to conquer Narnia by force for centuries — to carry out all these plans. Of course we as readers already know who and what this "Aslan" is, thanks to the first chapter. But even if we didn't, everything about him is so utterly contrary to everything we know about Aslan from the earlier books in the series. It makes one wonder — do Narnians in general not learn anything much about Aslan during their lifetimes, whether or not they ever see him? Do no Narnian scholars record and pass down the accounts of him and his deeds? Do all these poor creatures really have no idea whatsoever of what he's like?

And back to Tirian and his decisions — it's understandable that he and Jewel rush to Lantern Waste after the Dryad comes to them and dies in front of them as her tree is cut down. But that's right where Lewis tells us "how foolish it was for two of them to go on alone", and emphasises that it's their anger that keeps them from thinking more clearly about what they're doing. What if they'd thought more deeply about what Roonwit had told them — that Aslan IS NOT in Narnia — and realised there must be some terrible and dangerous treachery going on, and that, even if it means more Dryads will die in the meantime, they had better get hold of reinforcements before they go and investigate who's behind this?

Or even after they've committed their double murder and escaped — at which point they could have fled back to Cair Paravel and called a proper council of the King's supporters — well, I understand their crushing remorse and self-disgust. But why does Tirian then decide that the only thing to do is to give himself up to the Calormenes??? He's already seen that these people are masterminding the slaughter of the Dryads and the enslavement of the Beasts. Does he really think they're working for Aslan? Of course his object there is to ask them to bring him before Aslan. But he, as THE KING, has just handed himself straight over to his enemies as an abject captive. (What on earth would his great-great-great-great-great-great-grandfather [I think I have that right!], Caspian X, think of him deserting his post and his duties to ALL Narnians like that??)

Which is why, later in that chapter, when Tirian hears Shift's words and finally (!!!) realises "Ape... you lie", he's not able to keep on making the speech that Lewis tells us might have ended the rule of the Ape and broken the mesmerism that seems to have taken over nearly all the Beasts (except the poor Lamb) — because he's on his own, as a captive of the Calormenes, and they punch him in the mouth and throw him to the ground and tie him up before he can say any more.

I'm toying with what I suspect will be an Unpopular Narnia Opinion here, but honestly — the more I go over Tirian's actions in the first several chapters of LB, the less noble and heroic he comes across. I wouldn't say he is solely responsible for the doom that falls on Narnia; that's totally unfair. And of course, when all is over and he meets Aslan face to face in Aslan's own country, the Lion welcomes him unconditionally as the one who "stood firm at the darkest hour." (I may not have all these quotes word-for-word right — I'm away from home tonight and don't have the book with me.) 

But I'm going to put it out there: I'm coming to the conclusion that Tirian, far from being a wonderful and courageous hero, was a rash and reckless King who let anger and fear and despair cloud his better judgment; who totally deserted his higher duties to his subjects; who behaved stupidly and irresponsibly in multiple ways and did a lot to precipitate, or certainly to worsen, the disaster that finally overran and ended Narnia. And I am almost convinced that if he'd behaved more wisely — if he'd taken Roonwit seriously, if he'd asked for help from other older and wiser Narnians, if he'd put his responsibilities as King before his own emotional reactions, if he'd stopped and thought about what he obviously does know about the true nature of Aslan — Narnia might, just might, not have been destroyed after only 2,555 years of existence.

There, I've said it. Anyone up for a (friendly!) debate here??  

@courtenay I think you do have some valid points and I remember not being impressed with Tirian as a child/teen (maybe my first impression was the correct one!) but I keep thinking of one element of the series that has endeared them to me throughout my reading of them and that is none of the characters, besides Aslan, are perfect, they make mistakes, have flashes of temper, give into temptation, quarrel, complain, forget the signs, etc., and yet they still try to follow Aslan to the best of their ability, and as a Christian who is by no means perfect I find that encouraging in my very faulty attempts to live according to Christ's teaching. I think The Last Battle is probably the most Christian of the seven books and really has to be read in conjunction with the Book of Revelation.

In regards to how quickly Narnians seemed to forget Aslan I think Lewis was probably writing from an observation of how quickly society can change in our world. Perhaps the Narnians in the relative comfort and ease of the reigns of Tirian's predecessors had less reason to remember and relate the stories of Aslan and the Friends of Narnia than they had during the murderous reigns of the Telmarine kings.

It's kind of hard to say though I think the end of Narnia would have still come eventually. My pastor has been doing a sermon series for the year about Living In The Last Days, so it's almost something that's unavoidable.

So even if the end of Narnia didn't happened in The Last Battle, it still would have come to an end at some point, given to what Jewel says to Jill, "All worlds draw to an end, except for Aslan's own Country."

We could defend the talking beasts by pointing out that Narnia is a magical world and from their point of view, Puzzle-in-a-lion's-skin could be a disguise Aslan is putting on before revealing his true, terrifying form. It's not much of a defense though since the book implies that's what they believe lions actually look like. 

"Do you think it really is Aslan?" asked the King.
"Oh yes, yes," said the Rabbit. "He came out of the stable last night. We all saw him."
"What was he like?" said the King.
"Like a terrible, great Lion, to be sure," said one of the Mice.