bit late to the feedback party, but here's mine.
sometimes, the cursor gets stuck for no apparent reason, just by hovering over the terrain. when trying to place an item, sometimes it's not possible - perhaps indicate that placement is not possible before-hand (by changing the cursor to an interdiction icon or the like). because of this, it's kind of annoying to play.
it's not immediate apparent that you can rotate around if you hover the cursor at the edge. perhaps show some hints.
regarding the mechanics, i'm kind of like it, but i dislike the randomness of it. to test the castle defenders reactions, i've tried settings up a small defensive position comprised of 5 catapults and 5 cannons in front of them. in less than 30 seconds, the cannons were gone - they pretty much acted as very expensive mines. because it's not very clear where/how the cannons shoot, it's impossible to formulate a strategy of placing stuff around, so i opted instead to place mines on top of the attackers
it would be nice to replace the billboards for the cannons with real 3d objects, it would give the game a different feeling.