I don't see this listed anywhere, so I'll start a thread about it..
If you build your barracks too close to trees, units created will be either unable to get out, or will have to be led click, click, click every step of the way.
The AI on the most advanced setting makes this mistake also. I just played a game where there were knights unable to move, trees and other buildings blocking all the barracks the enemy had created.
My suggestion is to have it check, not just to make certain the ground is clear, but also that an area around where the units will exit is clear also. I think that should be easy to do. You just take the measurements you check for now, alter them when you place something, and then set it back after that.
Also, if when you are moving a building around to place it somewhere, it could change color, turning red perhaps, if that area is blocked? I put up a castle away at a differant gold field, and I have to wait for my peasant to walk all the way over there, then tell me it is blocked, despite looking perfectly clear. Bumps in the terrain I suppose. Just move it around and try again, no major problem, just something that I think would be nice to have.
