Do you think you could elaborate a bit more on what that counterattack was like? What was the weather like? It was all on the Oroku Peninsula, I presume. Any urban combat?
Background: The Japanese on Okinawa were determined to hold out as long as possible and inflict as many casualties as possible. This was to be similar to the defense of Iwo Jima. Suicidal charges and attacks by the Japanese defenders were forbidden. Artillery and ammunition were rationed and used sparingly. As U.S. forces invaded and started to capture a large portion of the island. Japanese moral started to plummet. Gen. Ushijima, leader of the Japanese forces on Okinowa ordered an all out counterattack to boost morale at the urging of the 32nd Army staff led by Lt. Gen. Chō.
Location: At this point in the battle, The Japanese defense was split in two. Gen. Ushijima held the southern portion of the island. He had lost contact with the North though the North was still putting up heavy resistance at the time.
In the South, U.S. forces were assaulting a heavily fortified position called Kakazu Ridge which was heavily fortified by Cave and tunnel systems dug into rock. The U.S. offensive against this very heavily fortified position stalled, and this is when Gen. Ushijima ordered an all out counter-offensive along the entire Southern front of Okinawa at night.
Theme: I wouldn't include any villages in this location. Imagine a rocky area with grassland instead. This area should also have a lot of craters and destroyed equipment. Fierce fighting has happened around this area over the last few days.
The Japanese could spawn in caves dug into a small rocky ridge and covered with machine gun nests. It is night-time. This is also an all out Banzai attack, close combat. This particular night saw a lot of close quarter's hand to hand fighting. The American positions would be a series of foxholes hastily dug in over the previous day as well.
Since this is mostly open terrain, you could give the Jap's access to artillery--lots and lots of it for balance. Artillery usage had always been rationed on Okinawa, but on this night, Gen. Ushijima ordered everything he had to fire in support of this offensive.