Hi !
You can enter the "main-area" of the bluff when comming through the MINE TUNNELS !!! To get there you have to take a other way out of the swamp (I think it was over the small bridge and then east)... from
the mine tunnel area you can "ride" down to the bluff.
And to the catapult: I just "klicked" on it, so that the arm comes down and then I "klicked" it again... the arm jumpes up and damaged the wall and roof of the building.. than you can climp on the arm into the "buildings area"...
hope that helpes
Silaya