I suck at drawing 3D with perspective, but with a cheap isometric projection you can draw open spaces fairly easy (ofc there are more perspectives, but for game map sketching, its pretty useful). The main benefit is that you have measured AXIS, so if you dont have the skill to draw a 3D stair, just remember that its a slope between a perpendicular plane.
You can draw 3D functions/shapes this way. There are special methods for some shapes. https://www.compuphase.com/axometr.htm
You could make it even more cheap by using aproximations to the isometric perspective (ex: use 30° from the horizontal line) and rejecting realist proportions: 1) Grab a square grid paper 2) Set a center point, this will be (0,0) 3) Draw a line segment from (0,0) to (2,1), this is your X axis. 4) Repeat the same for the other direction, the Y axis, from (0,0) to (-2,1). Now you have an horizontal plane. 5)Draw a vertical line from (0,0) to (0,2), for the Z vertical axis This is the "cheap" iso projection.
TL;DR When you have Axis, measures and ratios are your friends.