I had some similar problem - read topic like "Durlags Tower lv2 - wardstones?"
At the first time I think that training room, statues and wardstones open he way. If you pick up Kiels helm and go out all doors will close - in fact some times not all - I was able to go two times back from the 2nd level for shopping, but it was boring to reload and reload to get some of that doors open whe you came back. Anyway I think that the first level was the most complicated and had most of hard combats you can not avoid or use cunny tactic to finish easy (the 3rd level was really easy). So (except of dead npc) it is better to refresh after 1st level and finish with tower!
SPOLIER
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At fourth level there are 2 some kind of skeletal warriors - I dont know english names - (the same kind what in Degrodels house in BG - by the way this was the hardest fight in my BG1- not unseen rangers(?) nor wardens of doom(?)) but you can see them behind doors and kill them casting fireballs (through closed doors
) etc without beeing hurt
Zbyszek