Bubble Tanks Tower Defense 2 is the sequel to Hero Interactive's popular tower defense game. The game offers both open path and closed path options, as well as several maps and modes to go along with each option. Place towers on the map and choose their upgrade paths to defend the map's exits against an array of enemy types. Towers and upgrades cost bubbles, which you will gain whenever an enemy is killed. When a group of four towers are upgraded to the max, they will merge into one mega tower.