Masteria Expansions, Names and the proof for each

In that room may be stairs down into the Badlands (probably not).

Someone somewhere posted about catacombs underneath Affluent Tower. (Catacombs exist underneath the eiffel tower, after all).
Alternate entrance, maybe?

Or, thats the future entrance to a new subway that takes you to somewhere in Ossyria.

The catacombs are actually under the entirety of Paris little peep.
While there is a possibility of the Mesogears laying underneath a big portion of NLC, its most likely that its just under Bigger Ben.

The only catacombs were likely to see in the meanwhile is in or around the remains of the keep.

Concerning the tower, the theory most suggested is that when the tower opens fully there will be a portal to Versal near the top. Read the back stories if you want to know more.