Tower & Bridge - an isometric map in the making

[Scoopz]

Hey there cartographers, having largely wrapped up my last map, and because I want do more drawing, I have decided to focus my efforts on drawing the map of a tower connected to a bridge, or vice versa really, but whatever.

Anyways, having seen some of the one page dungeon challenges from past years, I decided I wanted to make this an isometric map!

I'll be using this thread to track my progress and ask any question I might have!

Here's the initial floor plan and the beginning of the wall being drawn:



Now, it isn't much right now, but it'll give you a good idea of what i'm aiming for even though I'll be deviating from the plan as I get to the top of the tower.

However, on to question(s):

Does anybody know a reliable way of drawing a spirally staircase? I'm using GIMP and I don't think I can get precise enough with my paths for it.