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Well, it's difficuilt to show, how something is missing, but here is my map:

http://imageshack.us/photo/my-images...earenamap.png/

The dungeon you see in the east is the hall of soar and appears just for some quest.

The Fortress of Ice should be somewhere where the cursor points.

Maybe there's a limit on how many dungeons the map can remember, and when you discover new ones after the limit the game just starts forgetting the oldest ones? Just a wild guess, I haven't experienced this.

It's probably the best guess, because i visited a couple of them recently.
Luckily, i still have a save state where the fortress of ice exists. It just has pissed me off, since it means a difference of two levels for me ...