You are a castaway and you're washed ashore an island. At first you think it is an uninhibited island, but no, there are creatures living here. Those creatures need a crystal to survive, but there are a lot of creeping, crawling and flying bugs that like a bit of the crystal as well. If they eat the whole crystal, the island and its creatures is lost. You can build defence towers that target a certain kind of bug. Certain towers only target certain bugs and they all cost money. By beating the bugs, you earn money and gems and you can invest that in more towers. Do what you can and save the island.
Use de arrow or WASD keys to move around, the space bar to build towers. The mouse makes it possible to select what kind of tower you build.