As the level progresses, gamers earn points for killing enemies and taking over control points from the opposition. Earn enough points and the game signals you that you can upgrade a unit. Each of the four classes starts each round as a level one, but can upgrade up to level three. The upgrades make your main weapon fire a little further and stronger, and upgrade your armor just a bit. Determining what order to upgrade each class, as well as determining what order to try and take over the control points is a large part of the game's challenge, as is finding the quickly respawning bonus items strewn throughout each level.
With fifteen challenging levels in the campaign and co-op play, Undertow earns your money with just the story mode; the best on Live Arcade. The fantastic multiplayer battles over Live are simply a delicious bourbon-soaked cherry on top of the sundae. With full sixteen-player support, the game never lets you feel alone, even at times when there may be one other person in the game with you. Rather than leaving a few Undertow obsessed fans drifting throughout the deep seas in search of each other, the game fills the remaining slots with A.I. bots. While many games fall apart when A.I. gets involved, Undertow's bots are the viciously evolved descendants of Pac-Man's ghosts. Unless you check the name of the baddie chasing you down, you would have no way of knowing you are not facing a real, skilled player.
The games controls are similar to those of Geometry Wars, with the left-stick handing your movement and right-stick handling the shooting of your primary weapon. The left-trigger's speed burst comes in extremely handy when using the smaller characters, as it never runs out and can be repeatedly hit for short bursts of speed to quickly get you out of harm's way, or around to the other side of a large enemy. The right-trigger is also useful at dispatching large baddies, though much more limited. Each time you respawn, you get three depth charges, with a maximum inventory of five for those who can find the mine-shaped bonus items. Pressing a direction and the right-trigger will lightly lob a depth charge into the surrounding water, wrecking havoc on anything within the blast radius.
Undertow is one of those great games where the demo and description do not do it justice. It is one of the most addictive, challenging, and deepest games Live Arcade offers.
