Treasury Chamber
Based on the number of Lunar Bees captured by the other kingdoms—he expected that the treasury chamber would have a lot of items. Not to mention, all of the treasures should be a high-rank one on average as the Honey Moon happened at the Second Breath.
Rex was expecting at least one Origin-rank item but that might be a stretch.
He would be happy to have a couple of legendary-rank items.
As opposed to having the treasury beneath the castle, or behind a secret door—the treasury chamber for the castle was located above, between the ceilings. In order to reach there, one would need to go to the hallway behind where Rex's parents were buried.
"Is it really okay for me to come?"
"It's not like you're a stranger, Liliya. Are you nervous because this is your first time in here?"
"I am. If you spent time on the street, you'd know that you're practically revered like a God"
Rex chuckled, there's no way he didn't know that with his keen hearing.