0---------0
|xxxxxx|
0-------0 0
* 0 = cornerpost
* spot for gate in bottom right
So yeah... turns out you can't do that. It must be the fact that the two wall segments are not co-linear, and it confuses some sort of check value in the game. I wish it weren't this way, but if there's a good reason to have things like this then i'll quit my whining.

If any of you weren't aware of this, i hope i spared you some trouble.