Now that's quite a compliment!

It was all done in gimp. I drew the top down outline of the map - all the walls essentially. Then I rotated it and scaled it vertically, and did the same with a grid layer. Then I chopped the map up where the elevation changes and moved those sections up and down to get the height differences and drew in the connecting stairways by hand. That gave me the outlines and allowed me to set up the floor and wall masks I needed. Then I detailed with the pen tool before colouring the map. It was interesting, and actually a lot easier than I had expected. I've got a few of this style to do for a new open design project so this was a great chance to pin down the workflow for an isometric map.

I love the fact that you get the chance to do a lot more with light and shade in an iso map. I wish I could have spent more time on the cliffs, and also on the furnishings - but real life has time constraints unfortunately.