You'll probably see the reason for the multiply blend mode if you go into your layer stack and change one or two of them back to normal.

Typically, multiply is used to turn white areas of a layer transparent, so the background texture will show through. It's pretty common with a stamp-based isometric workflow because if you're just using brushes, then all the white areas of the brush will be transparent. When you layer them on top of each other, you'll still see the lines from brushes at the back showing through. If you instead use a stamp where the background is actually white, though, you can layer things so that they occlude one another naturally. Then you set the layer to multiply so that the white vanishes, leaving only the lines you want to see.

I haven't read this tutorial, so I don't know if that's the case here, but it's the style of map where that approach usually works well.

It's also useful if you've got an element that you drew on paper and scanned. Just put it over your image and set to multiply, and you'll have wonderfully blended lines. When I made my first map, I tried to delete all of the white areas, and it did not work out so well. Plus it took a really long time. If only I'd known I could get the same result with a single switch!