I can't, actually - that's the problem. I've killed everything else in Source Square except the Engineer and the troll. I've found the levers, but I can't pull any of them. The Engineer's shield remains up. I'm probably missing something basic, but what is it? Any help would be welcome.

You can pull the levers, but they have to be the glowing ones. If you can't you might be bugged.

I have the same troubles. The levers cannot be pulled. Is there any kind of trick here?

no trick, move to the levers and you should see (e), you pull them and each turns off a light over the troll's door.

pull all four and the engineer's power source for his shield goes dead...then he's easy pickings.

OK. It seems something bugged and I didn't get the entry scene for the Square. No entry scene not way to kill the Engineer. Just load a previous save ( I had to load a normal not an autosave)