Tower defense games have to strike a difficult balance between familiar gameplay and innovative mechanics. Defense Grid 2 doesn't fix what isn't broken, either with the previous game or with the genre. Instead, it builds upon its previous success and adds nuanced gameplay touches. We participated in the limited beta, which only features two levels, and we found some significant depth to it.

As in most tower defense games, you are in charge of stopping a series of enemies from reaching the area you're trying to protect. In Defense Grid 2, these enemies are many types of aliens, each with their own attributes and behavior. The bulk of what you'll face is regular aliens that have no real strengths or weaknesses. There are also specialized aliens, such as flying ones that have great speed but low health. More worrisome are the giant tank-like creatures that plod forward despite taking an obscene amount of gunfire.

In Defense Grid 2, your goal is to protect a series of crystals around a reactor. From their spawn point, the aliens make their way toward this objective via the shortest path available. The maps are composed of a free swath of grid squares that you can build upon, and you can force the aliens down alternate paths that you've created. This allows for a high degree of player creativity, since you can make the path meander. This also lets you plot the best ways to maximize gunfire on the aliens.

The selection of towers in the game is similar to the expected genre norms. Standard gun towers are cheap and weak, cannon towers pack a longer-range punch, and flamethrower towers cause damage across an area. Laser towers damage over time, even after they're stopped firing at a target. Meanwhile, temporal towers slow down nearby aliens and keep them exposed to local towers for a longer amount of time. Finally, missile turrets have a low fire rate but a long range, and they inflict colossal damage. Towers can be upgraded, granting them increased attributes such as damage, fire rate and range.

Before you place a tower, you can place a boost block in a grid space. A tower built upon a boost block is elevated and can fire over nearby turrets. The second benefit is that the boost block can be upgraded to provide a permanent damage boost to the tower on top. This adds another layer of complexity to your tower-building strategy. You'll want to elevate some towers over others in order to have the greatest number of turrets firing at all times.

The difficulty comes when aliens reach their objective and take one of the crystals and carry it back to their spawn point. Granted, it's unlikely any alien that ran through your gauntlet of gunfire has enough health to make it back. When they die, the crystal falls and floats back to the objective. At any point, another alien can pick up this crystal and start carrying it back. This is especially problematic if you have a lot of traffic going in both directions on a path. You only lose the game if you lose all of the crystals. At the same time, it takes a great defense strategy to not lose any of them during the course of a game.

Defense Grid 2 has a few improvements over the previous game. For starters, players can rotate the camera to see the action from different angles. The game will support the Steam Workshop, and players will be able to create and distribute their own maps through the service. Concussion, Mortar and Tesla towers have been teased as a new addition, but the beta does not currently feature them. Finally, the game will have both competitive and cooperative multiplayer modes.

From our experience with the beta, Defense Grid 2 strikes a good balance. It keeps a true tower defense style of gameplay but augments it with its own twists. The beta was limited to two levels, so it's hard to get a good sense of what the final game will be like. As it stands, the beta remained fun for many playthroughs. This bodes well for the finished product, since it'll have more levels and the potential for even more offerings on the Steam Workshop. We can't wait to see how the game comes together when it's released next month.