Arc System Works dabbled in the defense genre with a DSiWare game called Castle Attackers. This isn’t tower defense though, it’s a real time defense game. Maybe it’s better described as a unit moving game?
Players draw lines to make the red troops move in Castle Attackers. You can target an opponent’s castle to attack or retreat to reinforce one of your castles. In the heat of battle, there are tide turning skill cards like a guard boost that protects your castles and a call to arms which recruits more soldiers.
Castle Attackers has a total of fifty stages spread evenly across five chapters. The game is available on DSiWare for 500 DSiWare points, but only for Japan region DSis.