When printed without resizing, the fort will fit 10mm figures. However additional details have been included in the design that will only show when printed for 15mm figures.
The logic scaling factor for 15mm is 150% (the one I used in all the print you can see here and on my Facebook page http://bit.ly/fbprtter ) but you may try a bit smaller (ie 140%) or a bit larger to see if that fits better your figures and taste.
Following a question about 6mm figures, I discovered the fort could also be used for that figure size. The scaling factor I used is 70% as it gave a better result than the more logic 60% (again see pictures on my Facebook page). Here again, best is to try with your own figures and see what result you like best.
At 70%, obviously some of the details are not visible anymore but the general look is still great (the desert themed fort is printed at 70%). At that scale you won’t be able to have moving doors and maybe other print that are too thin.