Last year, we said the title looked like Worms, and that's still true. While there is a single-player campaign being made to teach the game and its mechanics, the multiplayer was on display at PAX, and that features two different blimps (controlled by two people) flying around a 2D world with various floating islands and landscapes between two headquarters.
Each player earns gold over time, and they can use that gold to buy various units and structures. Outposts are the cheapest items; they're currently portrayed as floating crates with a colored area around them, and they can only be placed where the colored outlines touch. That's how you first lay down your territory: Drop a few outposts off, and claim some of the land on your map.
When that land is claimed, you can build other structures on it. Gold mines can only be built in specific places, and they obviously earn you more gold per second to spend. You can also build cannons, missile launchers, or lasers, which can be used offensively against the other person's structures. There are also defensive structures, like repair buildings, destruction team factories, or shield generators, which kick out circular shields wherever they're pointed.
The trick here is that you've got to fly your little blimp around the map to lay down buildings, fire cannons and lasers when they're ready, upgrade buildings and re-build shields when they go down, and aim everything at the best target possible. You enter a building from your blimp by pressing the right trigger, so by the end of the game, when you're either trying to take down the enemy headquarters or desperately defending your own, things get chaotic.
Turtle Sandbox says there should be over 20 levels to play through in single-player and online multiplayer, whenever the game is ready to go. It may be a while before we hear about Cannon Brawl getting its release date on PC and/or Xbox, but when it does we'll be there, ready to play.