First off, if you are thinking of buying this game, there is a demo on the Nintendo Channel, DS Download Demo service, on the Wii.
This game is brilliant. You basically have to build a fort/castle to stop the enemy intruding your town. After building it they come and destroy it, you have to also try and fight them off. Then, you have to "re-build" your fort and then prepare for battle etc etc.
This game is very fun, ever for the "un-hardcore" gamers.