Ok iv worked right 2 the top of the inside of the tower, and defeated the ghost, then iv gone to the top of the tower outside and killed all the basilisks and retrieved the sword , now what?

good! now you are all warmed up for the real test. durlags tower should actually be called "dulags basement" because this is where it will get tough (if you're not ready). anyway, go to the lowest level and in there you should meet a fellow thief (who has some words of warning) and in this room you should find a hidden doorway that leads to a lower level that later leads to another level which leads to another... all i'm going to say is that you better have a good theif who can find/disarm traps and unlock things or else you will not make it too far. IMHO "durlag's basement" represents about a fifth of the game and is a real challenge. have fun ;)

Make sure that you are really ready to go down below - you should be of a high level and well into the game.