Knights will hack and hack at those doors out of compulsion, only to have all their hard work come to a dead end. The only key is that the back end needs to end in team ladders or something so that it gives the logical conclusion that the doors go all the way through. For added hilarity, make a million doors. Used more as a distraction than as a sturdy, cheap, or easily repaired defense. Knights are attracted to doors, leaving archers and builders as easy targets. Repair the doors only as long as the other team thinks they go through, if/when they expose the dead end, fill it up with less-expensive rock and just use it as a normal wall.