Ive completed the first quest set by T´rang, but I cant figure out how to get into the castle, in the second quest. I mean, when go up with the elevator, only one door is open for me, and thats the way out. how do I get in????

You need to kill the assassin, right?

Have you checked out if there are any other ways to get into the castle?

How much of a spoil do you want?

Ive seen the catapult, but I havent figured out how too use it. I wanna get REALLY spoiled!

The catapult requires two clicks - one to wind the rope (use the star-shaped cog on the side) and then a second "hand-click" on the rope on the round timber. Then climb up and over the wall.

What he said.