Power towers is (was?) a custom tower defense map/mod for WarCraft 3.
The 'gimmick' in power towers is that your damage-dealing towers must be fed energy from generator towers. The game's difficulty is driven by the tradeoffs between dealing more damage, not running out of power, and extending your maze.
The map is heavily influenced by Cube Defense, with a focus on mazing and interacting with your neighbors. This stands out in the top-down view of the map: there are clear opportunites for mazes to take advantage of the terrain, and neighbors have nearby or even shared waypoints. The following image shows the top-down view, as well as the path Player 1's runners will take:
Towers sell for 100% of their cost, encouraging constant reworking of your maze. This is especially important in the higher difficulties. The maze shown below is relatively inexpensive and causes the runners to pass twice, but it won't be good enough for the next round on elite difficulty because in round 5 runners have bonus speed. Also, if player 8 was present, you would be splitting damage over runners that weren't yours:
Power towers also has alternative modes to keep things interesting: race, rush, and war. In rush mode, the next round countdown starts as soon as any player finishes the current round (instead of all players). In race mode, a player can start their own next round (even if the current one hasn't finished). For true danger, combine race and rush. In war mode, players can spend energy to send extra spawns to others.
There are also options for controlling the number of lives (sudden death, 10, 25, ...), the number of rounds (last man standing, endless, 15, 30, ...), the difficulty, teams (ffa, corners, sides, coop), and who gets income when a runner dies (owner, killer).