A rather simple defense game. In fact, you could argue that most of the game's depth ends up being unnecessary, since if you just repair the balloon whenever an opportunity shows up, you can get the achievement for escaping before day 4 easily.
Random trivia that I've found in the process of getting to day 7:
-The fort makes you untargetable by literally all enemies, even the flying ones and the ones that are twice the size of the starting fort. If you're getting overrun, just head up into the fort and you'll be safe. Enemies don't come back once they're off the screen so feel free to come back down in order to kill the stragglers.
-The machine gun is the best weapon in 99.9% of situations. The Boompipe can instantly kill everything in one shot, but the low fire rate results in poor crowd control, a weakness that the rapid-firing and fast-reloading (with the second upgrade) machine gun doesn't share. The MG isn't even that bad at killing the huge guys.
-If you're performing an action during which you can't shoot (repairing the balloon, healing yourself, building your fort, probably) while holding a rapid-fire weapon, you can hold the fire button down to cause massive lag. This is completely pointless and goes away in a few seconds.