It's weird because the unit can climb up onto the stairs, up the causeway, and into the gatehouse but cannot transition onto the ground the composes the upper level. Or, if you start on the upper level, you simply cannot transition into the gatehouse. I probably won't have time tonight, but I can get some other screenshots up to show off what I'm doing exactly. Most of the rest of the map is done at this point. Literally just this tiny issue holding everything up.
(Also sorry for the slow responses, holiday and such)
Thank you all for your input!
I wish I knew the answer.
I realized I had the exact same problem years ago but with a completely different object and was able to figure out that some objects in the game (or planes, rather) can only connect to those of other objects and not terrain. I don't know why, but that's how it is. What they do is add an invisible object that has a plane which does connect to terrain and put it on top of the object that doesn't.
To fix your problem: Find the invisible HelmsDeepRampart05a object in the objects menu, place it anywhere in your map, then copypaste the XY Pos, Z and Angle values from the normal ramp to this object.
PS. Be sure to show me your map once you're done!