I was thinking of different types of towers when i saw my brother's katana collection, wich made me realise Hack/Mine just NEEDED a Japanese type. These towers can't be very high (usually around 4 floors, 5 is max) but every single level of them is harder than 3 levels of a regular tower.
The levels will all have 6-7 spawners of the same type mob in it (prolly impossible level if the spawners are Raptors, Skeletons, Scorpions or Posion Slimes) and every layer will have a chest similar to a dungeon's. The final level will have an Epic Monster, the Oni, wich has to be beaten to access the chests. In japanese towers, the mobs and chests have a higher chance to drop or contain Shurikens and Naginata's than in other areas, and those weapons are also often powerfuller.