The Back Gate sits facing the enemy of the kingdom, Katsuren Castle. For this reason, Gosamaru took extra precaution when fortifying it.

The first thing we should observe is the technically advanced arched gateway. Usually, these arched gateways are made by flat rocks or a wooden frame.　Mechanically, arches become more stable the more pressure there is on the top. However, it isn’t enough to simple build the shape in an arch.The sturdiness of the stones on both sides of the arch must be calculated down to the finest details. It was Gosamaru who managed to pull this off.

Also, you may have noticed the staircase leading up to the gateway. In order to break through the back gate, one would have to break through the wooden door. And to do that, one would theoretically have to take a large log of some sort and slam it into the door like ringing a bell. However, with the gate built up the stairs, there was no way they could hit the gate directly from the front. You couldn’t get a running start and you couldn’t use horses to aide you either.

However, this feature was only added to the back gate of the structure. If this would have been done to the front gate facing Shuri Castle, it would implicate they were foreseeing an attack from the forces at Shuri. If you have time, try comparing the two gates again.