I'm having a weird problem. I've converted a couple of my crop lands into a corral, and am building a roundpole fence to keep in my animals. I've paved the space underneath them with red brick. When I go to build a gate between the two appropriately-spaced roundpole fences, I do get the option to build it, my character moves into place, but I get the red error message "you cannot build that there."
I *can* build two fence segments between them, though, and there doesn't appear to be anything in the way. And I have repaved the two tiles with stone to make sure that this wasn't the problem -- it didn't help. I have also successfully built a gate a good distance away on just a flat expanse paved with stone -- so it's presumably not some glitch with my character.
I will be gone for the next couple of days as I fly to the US to visit family for Christmas. I will experiment with other parameters and update this thread when I have a little more time.