I am stuck in Durlags tower lvl 3. i have been everywhere,but i can`t enter the secret door that leads to the next lvl. plz help!!! do i need a key, wardstone or something?

You need to be more specific are you talking about the floor where you have to find switches for the doors, and you fight all the Greater Dopplegangers?, or the one after that?

I have killed the doplegangers in the throne room and investigatet every room, but when i get to the secret door in the room with the false stairs it is locked, and cant be opened.

Ahh I see. I'm exploring the same area even now; however, I watched my brother in the same area. #1 Do you have all the ward stones?
#2 did you "open all the doors by way of the switch room? That may help but until I actually finish the area myself I can't be positive. I should have an answer before the day is out. That is if Six of Spades or one of the other veterans doesn't get to this post first. In which case you'll already have what you need.

There is a wardstone on that level which you haven't found yet, located in a room on that level that you haven't found yet. You don't need to fiddle with the door-switches, at least not that I can recall. Just send your Thief around, WATCH FOR TRAPS, and keep an eye peeled for hidden doors.

I may know where you are stuck. There is a room north of the "Throne Room" that looks like a torture chamber of some sort. Beyond that past a few traps that can't be disabled (which would severely tick me off if it was BG2) there is a room that appears to be a dead end. There is a container in the center of the wall in the middle of the room which holds a ward stone. When you pick up the ward stone a glyph on the ground glows. When you move your mouse pointer over the glyph it turns into the wagon wheel thing (you know says your gonna move from one area to the next. Click that and it takes you to the other side of the wall. Follow that path and you will evntually end up at the room with the stairs that lead nowhere. In the south of that room there is a secret door that was previously locked. I don't know the exact conditions met to unlock it but you should have acquired a new ward or special key maybe triggered a switch down the other path.

Yes Six, that sums up the problem nicely. In fact that goes for most of Durlags Tower.