Tunnels following walls and moats are quite normal and not mysterious at all. They just tend to be abandoned and forgotten after a few centuries of peace, as ur

Tunnels following walls and moats are quite normal and not mysterious at all. They just tend to be abandoned and forgotten after a few centuries of peace, as urban castles become palaces and city walls become an encumbrance, even if they had not been designed as highly confidential secret passages in the first place as seems the case here.