Spoilers for Season 5! How does Killian escape?

This is probably the saddest moment in this season and I'm so glad Killian didn't stay down here forever, but how did he get out of the bibliothèque dungeon (for lack of a better word)? Hades made it seem like the only way in and out was the elevator and there was definitely no buttons down there. I'm glad he got out so he could help Emma with Hades, but how? Did he climb the wall, cause there didn't seem to be many hand/hook/foot holds.