At first, the priority is to wall up, constructing a maze to get the attackers to clump up where they can be mowed down more efficiently. Then, the expensive towers can be built at strategic points, assisting the players greatly. A nice twist to the action comes when the sun goes down. At this point, all towers stop firing unless the creatures are lit up by the player's flashlight. Every once in a while, flares can be dropped that light an area without a player present, but during these nighttime waves, the player's role becomes less of a mobile tower than a spotter. It sounds easy, but if you don't watch closely, huge Berserkers come and break down the walls of your maze, which as you can expect causes huge problems. Eventually, the sun rises and things calm down again... for a while, at least.
Up to four players, local only, can play at the same time. The cooperative elements are solid, and you'll often find yourself and your friends teaming up at choke points, covering each other's backs. Detracting slightly from the co-op feel is the fact that each player has their own personal reserve of money; I'd have like to have seen a shared stash, or at least the ability to lend money if needed, especially early on when towers are very expensive. Another minor quibble is the fact that only a wall or tower's builder can sell it, which caused some issues with building the perfect maze. Thankfully, the waves are not continuous, so you can take a breather and coordinate as needed between rounds.
The best part of Murky Horizons? It's got to be the price, a mere 80 Microsoft points, or $1. Yes, for less than the price of a large order of fries, you get a nicely designed, very addicting game that allows up to four people to play at the same time. That's a quarter a piece, which is very casual friendly! Even arcade games back in the Golden Age were more expensive to play. As a mix of twin-stick shooter, tower defense, and with the nighttime light mechanic, Murky Horizon is quite unique, certainly worth checking out if you are even moderately interested.