It's a fantasy role-playing game.

DM: You are Vex Vex'ahlia, a half-elf ranger . You are the member of the famous group of adventurers. Previously, He and his party were left for dead, but his patron goddess, Tiamat, kept him alive. He offers a temporary alliance with Vox Machina against Vecna.
DM: Eventually, you come around one cluster of exterior buildings. It looks to be what was once a manor, that was converted to a fortress. Like a small exterior fortress to this inner circle of Thar Amphala. You curve around and there you see, familiar to you from once before, the walls that surround the exterior of this inner sanctum of Thar Amphala, and the basis of this field that contains the entirety of the city. Looking at the wall, you can already feel the dangerous vibration of this field being so close. There's this violent hum to the air, that with each pulse hits this certain frequency of sound that causes your ear to tweak just a hint. And you can see violent waves of green-blue magical energy, (pulsing) up the side of the wall, like a very powerful reverse waterfall. Shooting up, to gather and maintain this field. You cannot see the crest. The crest was visible from the outside of the wall. And the energy field is being produced on the outside of the wall. You're on the inside of it. But you can gather the base location of where this burst is happening to get an idea of loosely where it might be.
Player: How thick is the wall?
DM: You don't know.
Player: And we saw the crest before because there was a doorway in the outer wall.
DM: Correct. Yes.
Player: Is there any kind of doorway around us?
DM: You glance over to where the door was, below the crest, before, and you can see the opposite side of a door, currently, yes. It is shut. And appears to be made of hard wood with-- It's iron bar reinforced crosshatch.
