This is a tower building game for the PC and XBLA. You build a few starting towers and as you kill aliens you get credits to build and upgrade more towers. Your job is to keep the aliens from making off with all of your energy cores which is quite easy, it is much harder however to keep them from taking any at all.
There are many different tower types that are stronger against certain aliens or slow them down or boost the credits you get for killing them. Upgrading them is also a major part of it but sometimes it can pay off to have a few upgraded and a few fillers. You can use cheap towers to control the flow of aliens. You might want one wave to go down one path but another wave to take a different path. You can even bounce them around for a while if you need to, as long as there is a viable path to the exit they will change course over and over. But if there is no path they will just walk right past any obstacle for the shortest route.
There is also some back-story and a pretty cool narrator/guide but this is mainly a fast paced action experience. I've played it over 100 hours for a low $10 price they also have extra content packs at only $1 a piece. It's a rather simple game without a lot of content but it is pretty fun and repayable especially if you want to try to beat the high scores.