So this is something that's been bugging me for quite a while, and that I've been contemplating on and off. I should warn you, a lot of text incoming.

Because it's just downright strange. Consider:

The ruins of a Great Gate outside the city.

The barricades and the survivors of Kvatch.

Hirtel's raving about something coming over the walls.

The Oblivion Gate just outside the city.

To me they all seem to suggest a specific progression of events:

So the Mythic Dawn knew Martin was at Kvatch, but apparently couldn't identify him, or they'd have sent assassins. There's no other explanation that I can think of for the attack on Kvatch. Fair enough. But why the Great Gate? The walls and city gates are still standing, and if they'd brought a siege crawler like they had for Bruma they'd have had to level something to get in, and they would've trashed the whole city in the process. So it couldn't have been one of those.

Then there's the issue of Matius and the survivors who fled the city. But they would only have survived if the Great Gate was already closed, and the lesser gate before the city wasn't there yet. So if you've opened the Great Gate to destroy the city, why close it before you're done? The only reason I can see is that something must have gone wrong, and the Great Gate collapsed. But something still caised the damage in the city, some pretty significant damage. So what?

Hirtel says "There was a huge creature... something out of a nightmare... came right over the walls... blasting fire" Now while he's undobtedly an unreliable source, he seems to be the only one alive who saw whatever it was that did so much damage. Something that didn't need to go through the walls to do it. Mehrunes Dagon perhaps? After all, he did destroy the city of Mournhold, so it could be that he'd try something similar at Kvatch. He came through the gate to destroy the city personally, but for some reason was thrown back, taking the Great Gate with him.

Which brings us to the lesser gate, the one that sealed off the city. It's presence makes no sense. Why attack the city, only to damage it, then withdraw and seal it off? This certainly suggests failure, and an attempt to salvage something from it. This also explains why Dagon didn't take a hand in events sooner, since he'd already found that he couldn't act personally, forcing him to work through the Mythic Dawn, until Camoran's death forced his hand.

So that's my theory. Anyone else got one? A different take on events? See a flaw in my reasoning? I'd love to hear what any of you guys think.

That's interesting! I've never thought about it. But you have a point about the failure.

Though I wonder, why did this thing/creature/Dagon fail? Maybe because Martin was inside the chapel and had divine protection? That's all I can think of. But OTOH the slaughter in Anvil suggests that chapels don't offer protection against an evil power.

In my mind it was a test. Kvatch drew the short stick and the first "invasion" took place to see what they could do, how difficult would it be and could Dagon take a step through, if so how far. I think the great gate became unstable and Dagon had to retreat. IMO it was a successful failure? If that makes sense. They tested the water and found out what could/could not be done and went from there.

Well, I'd consider Martin to be a decent source, much better than Hirtel. He said "I was at Kvatch. I saw the terrible power of the daedric siege engine."

Makes sense too.

So in that case, Martin being in Kvatch was a coincidence?

I'm guessing that the Liminal Barriers were still too strong to allow Dagon entry for any length of time, though it could well have been Akatosh. Martin being present gave him a link that wasn't present in Anvil.

Considering how important it was to kill Martin, and the amount of planning that had gone into it the whole scheme, I'm not so sure it would have been a test. Or at least not originally intended as a test. Dagon may have gotten over-confident, or simply impatient, and decided to try and do it himself.

But the problem is there's no real evidence of a siege engine being used. For Martin to have seen it it would need to have been inside the city. The fact that there's no wreckage like we see at Bruma suggests that if one were used, it was removed before the city was destroyed, and taken through the Great Gate. Which doesn't make any sense.

I was about to post this.

It seems to me that the "creature" that came over the walls was the siege engine itself. It spits out a volley of fireballs and has giant crab-like legs, which explains the crawling and everything set on fire. Though I wonder what became of it. Maybe they took it back to the Deadlands for round 2 at Bruma?

The Siege Engine is constructed exactly like an Oblivion Gate, using a sigil stone as its source. One assumes that if it were shut down normally, by simply removing the sigil stone (before it was damaged by a collapsing Great Gate) it would simply collapse and disappear as the gates do.

Good one glargg - I'd forgotten about that line. It's been years since I played the main quest.

Edit: Scratch that.

But it still has them leaving before the city is trashed, for no apparent reason. I just can't see a reason why.