There is not much to see in the Mie Castle Ruins area, but being in the area, I had to stop and take a look around. It was on a hill and had a small shrine area and an open flat area about the size of a football field.
Mie Castle was a coastal fortress built in the second half of the 16th century. Along with Yawaramori Castle on the opposite shore, Mie Castle controlled the narrow entrance to Naha Port during the Ryukyu Kingdom era. They had gun loops for small-bore firearms and some large-bore cannons mounted in them as well. There was also a chain, which could be strung out between the two fortresses, to block ships from the entering the harbour. Nineteenth century paintings of Naha Port, such as the Ryukyuboekizubyobu (琉球貿易図屏風), show Mie Castle was connected to another smaller fortress called Naka Mie Castle (仲三重城) by a stone bridge. This is in turn was connected to Rinkaiji Temple (臨海寺) before another stone bridge linked it to the main part of Naha Port. In effect, it looked akin to a long breakwater with a temple and two coastal forts. Naha Port was connected to Shuri Castle by a military road built in 1522 by King Sho Shin. In May 1609, these coastal fortresses successfully prevented the Satsuma fleet from entering the Naha Port, forcing them to land their soldiers further north up the coast. The fortresses suffered heavy damage during the Battle of Okinawa in WWII. Yawaramori Castle was completed demolished by the Americans post-war when they built Naha Military Port, while the area around Mie Castle was landfilled and built over, with a modern hotel situated right next to the remnants of Mie Castle.
If you are interested in Japanese Castles, check out my Castle Page, with links to posts on the castles I have visited.