Start with a line.Hold and steer to draw connected 90° turns.
G flow, D draw, V select, H pan, F fit. In Select, drag a box to select walls; Shift-click or Shift-drag adds to selection. Ctrl or Command A selects all. Arrow keys draw or nudge. Shift and arrows extend a selected wall. R rotates right; Shift R rotates left; Ctrl or Command D duplicates. Escape cancels. Use Help for all shortcuts.