Rettoujin no Maken Tsukai: Skill Board wo Kushi Shite Saikyou ni Itaru - Vol. 4 Ch. 47 - The Hidden Room

[MangaDex]

 

[KyuubiUzumaki]

Watch it be a passage to some kind of treasure hidden in a stinky place. Seeing as he went below the surface, the city probably has sewers somewhere. I would not be surprised if the Royal Palace had a secret passage leading to the sewers.