An easy to follow guide for the Tsuki Castle Fortress Locked Space on Ashika Island.
Tsuki Castle Fortress Locked Space
There are 2 ways to gain access to Tsuki Castle Fortress. The first is to obtain a Key, which relies on a little bit of RNG luck on your part but they can be found.
The second way is to kill the Wheelson that patrols the castle walls. Once you have destroyed it you can hack the blow-up robot and gain access to the Fortress.