GeoDefense Swarm is the follow-up to GeoDefense, and is an extremely challenging open-path tower-defense game. Like its predecessor, GeoDefense Swarm stands out for a unique play style that mixes frantic arcade action with bang-your-head-against-a-wall puzzle-solving. Aside from the new open-path format, GeoDefense Swarm has much in common with the original GeoDefense, from its psychedelic vector graphics to its drag-and-drop interface and the same selection of upgradeable towers (GeoDefense Swarm adds a sixth tower, the Thumper, which deals damage in a wide area). You're still destroying a set number of waves of geometric "creeps," all with differing health and speed, and GeoDefense Swarm occasionally mixes up the playing field with obstacles and squares that have special effects, such as healing and speeding up creeps.
GeoDefense Swarm features 30 levels separated across three difficulties--Easy, Medium, and Hard--and you can still choose Novice Mode if you want to play without posting high scores. Because of the more open-ended possibilities of the open-path format, the level design of GeoDefense Swarm doesn't feel quite as tight as the original, and you might find yourself having to experiment even more to find winning strategies for the game's constantly evolving maps and enemy configurations. Depending on your personal preferences, this can be a feature or a bug, but GeoDefense Swarm is definitely more difficult than GeoDefense. Newcomers to tower-defense games may find themselves struggling, and unfortunately the game doesn't make learning the rules easy, revealing only incremental tips at the start of each level and piecemeal hints after losing levels.
Overall, GeoDefense Swarm is a solid, impressive game, an excellent value, and an especially satisfying title for tower-defense power gamers looking for a challenge.