A round yurt diameter platform on top of a square deck will work OK. Anchor the yurt cover to the round edge banding that extends up an inch or so higher than tthe actual platform. And make sure your square deck can drain. Gap the boards. Insulate between the
framing before you apply your decking.
Keeping your yurt up on a dry platform has many advantages over merely erecting it on the ground. I built a low platform.
To see my deck, Google images:
Bob Rowland homemade yurt
Mine is the rustic canvas yurt with green door and rafters either side. That will show numerous details.