How do you unlock the tunnel cap on astro knights island?

Answer 1

You need a key from the secret society that reveres Mordred.

To get the key:

1. Get the secret message from the Maid's room (castle first floor, to the left)
2. Take it to the girl at the mill (upstairs) to get the password
3. Go to the fountain, on the brass plaque. Press the 3 symbols and the sun.
4. Get the key from the hooded fellow at far right.