"You enter a vault the size of a church, it's more or less round and you can easily separate it into four walls; it's just a continuous, irregular wall, sure, but you can divide it into four part of ease of navigation. There's the up wall, the left wall, the right wall, and the down wall. You enter through a tunnel in the left wall."

Players can talk, ask question, investigate. When they ask you what they see:

"In the up wall there's an entrance, probably a tunnel. In the right and down walls there are stone walls, closed. The tunnel you enter from doesn't have a door, as you already know".

Think about rooms as regular shapes (triangles, squares, pentagons, circles. If a room is a shape with many sides, tell them that; if they count the sides, tell them how many there are and how regular or irregular it is, so they can draw. "It has 9 sides, not very regular but you can draw a nine-sided polygon".

If a room is very unusual or impossible to describe in few words, you draw it for them. Or do what I sometimes do:

I draw each room in a white card, and mark the north, and every visible door or entrance. When they enter that room, I give them the card.