OK....I got to Marten's Bluff. I found the catapult, cranked the wheel on the right side and fired it toward the wall. I walk up the catapult arm and onto the wall. However, only one of two things is happening. I either end up falling onto the roof and can't get out again, or I am unable to get past the corner of the wall. I move my party forward to the arch, but just past the corner where I walk up the catapult arm, the party is unable to move onto the arch and gets knocked onto the roof and trapped....Sigh. How can I get into the fortress to get to the T'Rang arm?

you don't have to go onto the arch. Go on the roof to the right of where the boom lands and you can go down a ramp into the inner yard.

Hiya SkyRanger,
Welcome to the board! Here's a spoiler that you may or may not wish to look at:
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You will probably have better success in Lower Marten's Bluff if you get there via the Mines. There is a pulse pick on the east side of a door that you need to open said door. And guess which side you start on if you go down from Upper Marten's Bluff? The west side. Alternately, you can start quest #5 or so of the Umpani. You can get the the mines from the Northeast corner of the Swamp.

Thanks. I completely missed the ramp going down into the courtyard. D'oh! I gotta quit playing this game late at night. LOL.