Okay kiddo, so Tenshu is a Japanese word that means "castle keep" or "main tower". It refers to the tallest and most important building in a Japanese castle.
The Tenshu was like a big, fancy house on top of a hill with lots of rooms for important people to live in and do important stuff. It was usually made of wood, but sometimes it was covered with plaster or tiles to make it look really cool.
Inside the Tenshu, there were secret passages, hidden doors, and trapdoors to make it more fun and exciting. It also had thick walls all around it to protect the important people inside from enemies who might attack.
So basically, the Tenshu was like a big, tall fortress with a fancy house on top where the most important people would hang out and do important things. It was like a really cool and safe clubhouse for important people back in olden times in Japan.