Don't let the creeps reach the middle. Every creep reaching the middle of the platform will spawn a heavy particle beam, making parts of the platform unusable. Once you lost 50% of the platform, the game is lost.
Use different types of towers with different ranges to build a maze. The longer the creeps need to reach the middle, the longer you can shoot at them. Survive the waves of creeps and progress through the levels - with each level your platform will grow bigger and will spawn more creeps, which get more powerful as well.
DeHexTD will keep you engaged during the waves. Keep expanding the maze, upgrade towers and find a good balance to survive as long as possible.