Don't let the ghosts kill all the chicks. Move with arrow keys and shoot with space. There are four modes. In infinity mode, ghost spawn every 5 seconds. Also, the chicks do not appear like they do in waves mode. In waves mode, every time you kill every ghost, the next wave spawns. The next wave has about 2 more ghost than the last wave. In creative mode, you can't die and can spawn chicks ghosts and place trees by clicking on an icon and then pressing g. You can also reset the map by pressing g. In hardcore mode, you only have one life, and ghosts spawn more often than in infinity mode.
Update 1.1 Added creative mode. You now shoot clicking space, not by clicking your mouse. To place a tree in creative mode, click the block in the right, then go with your mouse where you want to place the tree and press Q. To spawn ghosts and chicks, click their icons, and they will spawn on the random position on the map. Update 1.2 Added Hardcore Mode. You only have one heart in this mode, and ghosts spawn every 2.5 seconds. Also, now in creative mode, you can reset the trees by clicking g.