I'm only about 30 days in (the game is tracked by days), but here's my impressions so far.
Like a modern Rampart, during most sieges you alternate between a timed build and battle phase. There are 4 types of objects you can build; walls (includes gates), turrets, traps and helpers. For the 3 latter objects, you need to obtain the appropriate scrap in battles and assemble them before being able to play them. Pretty easy. Most battles you need to carefully arrange defenses to protect key people or places, though there have been some different objectives. During battle phases you balance a combination of engaging enemies directly and repairing your defenses. One other game mode I've encountered so far is similar to a lot of flash games you find online where your tower is on the right mounted w/ a gun and enemies approach from the left. A much simpler game, but can be used to help you stockpile Source, the currency for building and repairing.
Overall I'm having a lot of fun with the game, but here's what I feel the game's missing:
Camera rotation. Was Final Fantasy Tactics for the PS the only isometric view game that had a rotating camera? Why can't we have that again? This is something I find lacking in a lot of DS games. Seriously.
Better path finding. Too many times have I had Lock take the long way around when I put gates leading through the middle of my ramparts to make it easy to get from end to end.
More precise object/unit selection. This may be tied to my complaints on path finding. Sometimes Lock will choose to walk to the outside of a wall/turret to repair it, even if the inside is closer. Also, because of the fixed camera, it seems like it can be difficult to differentiate when selecting a patch of ground to move to or a wall/turret to repair.
Even with these complaints, I still rate the game highly for fun and enjoyment. If you enjoy tower defense with a little bit of storyline, you'll enjoy Lock's Quest.