how do u open the secret door in the keep dungeon?

buy the key in the tavern from the dwarf

If you are refering to the treasure room in the Halls of the Hammer then you need to buy the key from the dwarf in the Purple Wyrm for 4000 gold. If you are referring to the secret room in the Keep of Pale Night Dungouns, where you rescue the original three, there is a loose stone in the NE corner you need to push.

On a side note- If you rescue the original three in a certain order with the different characters you will geta special weapon. I only know of Dorn so far. If you release Vahn first with Dorn you will get a really sweet crossbow as opposed to the standard +5 crossbow.

but if you release the dwarf first you get a kik a$$ axe, it is awesome!!1

which 1 is Vahn??

The archer