Underwater Defense Wall
An Underwater Defense Wall is a mini boss from Wario Land: Shake It!. Guarding the Merfle Barrel in each of the three Subwarine levels, this guardian attacks by generating enemies and switching on environmental features in an attempt to defeat Wario and stop him from finishing the levels.
There are three types of these mini bosses in the game.
Defense Wall System 1
Located in Wavy Waters, the first defense wall system attacks by generating submarines and diver Bandineros. To defeat it, Wario must shoot the red button that appears in its core with missiles, which will eventually cause the machine to explode and let Wario rescue the Merfle and finish the level.
Defense Wall System 2
Another similar system appears in Windbreak Bay. Again located at the end of the stage, this version has three cores that Wario must destroy with missiles, and uses underwater fans to try and push Wario around in an attempt to make the battle more difficult.
Defense Wall System 3
The final such system is located in Creep Blue Sea. Just like the one in Windbreak Bay, the defense system here has three cores that Wario must destroy with missiles to progress, but now also has spikes littered around the floor and ceiling. Once this version is defeated, Wario will encounter no more Underwater Defense Walls (or Subwarine levels in general) for the rest of his adventure.