The Abandoned Church seen in the episode "Crawling the Catacombs" was built with a secret passage to the Paris Catacombs in the floor, underneath the bell tower. Dante Vale, Zhalia Moon, Lok Lambert, and Cherit find it using hints that they found in Joan of Arc's notes from the Cavern of the Gargoyle. DeFoe and his Suits attacked them, and they retreated into the caverns destroying the entrance.
The bell tower had sunlight streaming down to the floor, over the entrance into the catacombs. The sunlight streamed from a series of mirrors that reflected it. The mirrors were the key to opening the passage, because a spell (Sophie used Raypulse) had to hit it from a certain angle that the mirrors would put it in.