i transmigrated as a prison guard and suppressed the princess

Xiao Ran transmigrated to a fantasy world. As a dungeon guard in the Celestial Dungeon, with a special floor called the Purgatory meant for powerful demons and martial arts experts, he faced a huge challenge from the start. The Eldest Princess acquired a forbidden martial arts technique and was banished to the Purgatory. However, no one was powerful enough to restrain her. All the dungeon guards were defeated. Xiao Ran stepped up and declared, “Your Highness, it's better for you to behave yourself on my territory” and effortlessly subdued the Eldest Princess.
He progressed from imprisoning the Eldest Princess to delivering the Great Xia Kingdom from the evil clutches of various demons and evil martial arts experts.
...
Xiao Ran was the mastermind behind the scenes.