Yasama essentially lets Japanese Towers act as miniature Castles. As it is available in the Castle Age, it can be used to follow up a Tower rush (trush), as these involve flooding the enemy base with Towers in the Feudal Age. It is also an excellent defensive technology. Combined with the other Japanese unique technology Kataparuto, which allows Trebuchets to pack and fire faster, it makes for a powerful defensive combination. Trebuchets are already a great defensive tool in the shelter of a Castle, but adding in several Towers can make a huge difference. They can even be used without Castles, although it is still significantly less painful (and far faster) to swarm a Tower than to swarm a Castle.