A silly tower defense game. Fortunately, I happen to like silly tower defense games.
It offers a decent variety of towers and enemies, always a key ingredient. (Although I found some of the tower types more or less indispensable, and others pretty worthless.) You get money from killing enemies, but also from building generators, so there is that tension between investing and beefing up your defenses.
Lastly, the game brings us something new: orbiting towers. All your defenses must be placed at preset points in 2-4 rings surrounding your ship/station. These rings are fixed in distance from the center, and the build points are fixed in distance from each other, but the entire ring can be rotated. This lets you move your towers into position near the enemy's attack paths (highlighted by glowy dotted lines), or even follow a wave around to keep it in range longer. Since the levels are in space, with no real 'terrain', this is a good way of doing things.
One of the game's strongest points is actually not the system itself, but the whole aesthetic. The entire thing, down to the last detail, is done over to look like a cheesy, low-budget, 40s/50s sci-fi movie/TV show. The costumes in the live-action cutscenes, the flying saucer enemies, the robots that look like they were patched together from old car parts, the towers that look like they're made of cardboard and tinfoil, and the buttons & iconography on the menus all appear just like you'd expect them to. I've never played a game that did something similar, and while I wouldn't want it to become a trend, it really works here.