Looks like I made the map a bit confusing... if you have any problem, take a look at this guide.
To make the fight againts the vampires easier take the double sword and the shield located in the great crypt before going for the power potion.
After killing the orcs (there's a bow and a quiver in the dead man's room) "use" the altar. Kill all the skelletons. Look for a crack in one of the walls (it has some smalls stones on it, the character will look them) . Add the chisel of the dead man's room to your inventory (it should happen when you take it). Take the hammer that's over a gravestone. Go back to the crack's wall and use the chisel. Hit it 3 times with the hammer. Get the ancient skull and use it in the altar. There is a hidden potion in the big barrel of the dead man's room.
Take the torch that's on the second floor and use it with the lamp in the dirst floor. Go to the rope and use the lighted torch. The minotaur is killed, so you can enter the cathedral..
There is an invincible knight in the way to the stairs (he can kill you with 1 hit). To pass him you have to kill the traitor that is in the middle of the cathedral and use his cadavre (youll dress up with his armor). You also need the map that is located behind an iron gate. Kill all the knights of that floor (not the invincible, of course) and it will open. With the traitor armor and the map the invincible traitor will let you go upstairs.