An RTS, huh? That sounds too complicated for the DS.
Not likely, disembodied voice. While the gameplay is much more simplified than your average PC real-time strategy fare, with almost all input done through the touch screen, it still induces the high blood pressure intensity associated with the genre.
You start with an empty city on screen, with several plots of clearly developable land and the resources to build Dojos, which train / create new ninjas to protect your town from the oncoming waves of demons. Placement is very important for your Dojos, as ninjas can't stray too far from the one they trained at. Thus, if a group of Wee Devils get away from them, you'll have to build a new Dojo quickly on the other side of the village. Let the Devils get away completely and they'll attack and damage the center of Ninjatown (ProTip: Don't let the HP reach 0.) Defeat the Devils and you'll receive more resources like Ninja Cookies, which can be used to build more training facilities. No mining necessary.
What type of ninjas can defend this town?
Each ninja has their own unique strength to take on fiends. You'll recruit basic Wee Ninjas in standard black, orange Anti-Ninjas who prefer brute force to stealthy agility and White Ninjas who can freeze enemies with the power of cold, just to name a few. The Wee Devils employ many different types of units as well - some fly, some are super tough, etc.
The strategy comes from the variety. You need a unique set of ninjas and that comes from carefully choosing which Dojos you construct and whether or not to spend more money (err, cookies) on improving them. You can't make the buildings tougher, but you can give the ninjas who reside there more health and abilities.