I've seen some sieges work pretty well, ladders..siege towers, and all. And I've seen others with completely strange results. On my siege of Omor, if I placed the siege tower on the left side, they would push it up to the wall and not use it at all. Then all run over to the right side and stand doing nothing at the ladders. If I put the siege tower on the right side, it would go flawless and they would rush the tower and take the wall.
In battles where I'm outnumbered, I've used the circle formation and it went really well and was quite exciting, a very thrilling battle. In other battles I've used circle formation and have been left baffled by the behavior as the troops can't seem to figure out how to form a circle and run all over the place..