Lock's Quest is kind of an odd "Tower Defense" like action/strategy hybrid, wherein the title hero is tasked with constructing defenses around various playable areas designed to hold back attacking armies for a set period of time. It's actually a pretty solid DS title, and certainly feels unique, but it also tends to drag on a bit longer than necessary.
In Lock's Quest, you play as a young man named Lock, with dreams of becoming one of the world's Archineers, a play on engineers who use an energy called the Source to build various constructs and buildings. The Archineers are also a military force, and were apparently instrumental in pushing back an evil known as the Clockwork Army quite a few years ago. As most evil things tend to do, the Clockwork Army returns, and this ends up putting Lock on the fast track to becoming an Archineer, and throughout the game there's quite a bit of story involved, not all of which ends up being too engaging.