I'm not sure if door blocks are any more resilient than normal blocks, but it certainly seems to have a psychological effect. When most people are faced with a wall (early game without kegs) they try to scale it. Likewise some teammates are gifted in that they think trying to run outside to kill a knight as a builder, while he's standing right next to the door is a brilliant idea. Thus letting him in. While building ways back in without doors can be relatively tricky, it can be done and once the team is used to it it seems to work better. Knights always have to climb, thus attacking from above for greater advantage, special players have less ways of inadvertently letting enemies in, and until kegs start flying around they are less inclined to try attacking the wall itself. But, of course doors on ground level means our guys getting in, out and through faster. A way to climb back up the wall won't necessarily work while holding the flag. So doors on outer wall, yea or nay?