It is easier to use the Color Key sheet effect.

Fortunately, I happen to be working on something similar, so I can show you some screen shots.

The visible floor here is on the FLOORS marine sheet. There is a Color Key sheet effect on this sheet.

The Color key knockout colour is magenta, so when the Color key is activated the magenta cuts a hole in the floor to reveal the underlying sheet - one called INLAY BRONZE. There's a polygon of a bronze texture on that sheet.

So all you would need to do to adapt this to your own map is to add the Color Key to the sheet where you want to cut the holes and draw the holes as separate shapes in the knockout colour.

Good - but I think the trees could do with some shadows?

Thank you both for your ideas :)

I have to start Darklands City soon, but I think I might have to find a few more hours to do some more on this.

I have some nice Celtic knots I did for the wax seals that might look good, and then there is always Igdrasil.

Elementals too, I should think - symbols for Earth, Fire, Air and Water?