Okay -
When you enter Skull Castle there are stairs ahead of you (with a gate) and exits to the left and right - GO RIGHT. At the furthest rightmost wosname there are stairs going DOWN - GO DOWN. There is a small landing and the stairs continue to go down - GO DOWN SOME MORE. On the far side you will see stairs going up - don't go there. Look to your LEFT - ah ha! there is the gate that you need to open!
BUT FIRST YOU NEED TO TALK TO THE LICH
To do that - go to the tilting bridge and fall into the water. Take the western most north water passage to the room full of traps (lots of traps). On the far side of the room there is a table and key on the top (I had to jump onto the table to pick it up). Get the key!
NOW
Go back to the entrance and go straight - up those stairs and open the gate. You will be in a big room with lots of ghosttype creatures around (they'll leave you alone). Head to the far side where you will find a set of stairs going up (double stairs). Exit through the door and continue down the hall until you come to stairs going down - GO DOWN. Use the key you got in the trapped room - and there you will meet the lich!!
And if you are still lost, check Xignal's great maps:
http://www.volberding.com/WHints/index.html
Wyv