ToME: the Tales of Maj'Eyal

It seems that High Peak levels 12+ don't start you off on the stairs to the previous level. Is this normal behaviour or...? I didn't check to see if there are any stairs down at all given how I've been rushing directly to the next level (and clearing any vaults I happen to see) what with each level being pretty large, y'know.

When does it end exactly? I'm currently on level 15, is there some sort of gimmick I'm missing here? Should I have explored each level fully?

Edit: Well, found the portal. The boss fight there was surprisingly fun, and finally justified my decision to keep several sets of resistance gear on hand, even if I had to drop a fair bit of it mid fight there.

Once you enter high peak, that's it. There's no turning back until you win the game or die. There is also no warning of this, at all, so it's not surprising you didn't notice until a bit into the peak.

The peak is indeed 15 levels long, and the final fight is accessed via a portal on level 15.