The last time we saw a doorway like the old man's in HW2, it was the door where Glitchtrap was locked away behind. During SB, Glitchtrap possesses the Mimic's body, and controls everything from below. He has taken over the Mimic's empire, and it's metiphorical crown. (+)

FFPP, where the Mimic resides, also bares a very strong recumbence to OMC's realm. The old man very clearly being some version of OMC. (+)