ToME: the Tales of Maj'Eyal

Never had this problem before- on my undead archmage im on the way to the sher'tul fortress for the first time, but the staircase to the fortress in the water dungeon prior says I need a key, and I can't use the stairs.

Do undead not get the fortress and transmogrify chest?

All classes and races get the fortress. You are missing one thing, written in white text below to avoid spoilers in the general forum:
You need to pick up the lore item on Old Forest 4 that has the "key" in it to activate the quest. It will still be there when you return.
end spoilers.