Topsoil removal before starting??

Hello everyone
I'm starting my first earthen floor project. It's going to be in a geodesic dome outside my house.
Right now the dome has a cover and is standing on the ground as it is.
My plan right now is to make it flat, pour in a layer of gravel and then pour the subfloor, and then floor layer and finishing.

I read in a book that "at the very least" the topsoil should be removed because its less stable.
Should i do that or can i just flatten the surface with the topsoil and go in with the gravel?

Thanks alot!
Oren

Remove it. Top soil has organic matter in it that will break down over time and make your floor sink a bit.

You want to remove all "top soil" and preferably get down to the 2nd horizon of your soil base (this is usually referred to as subsoil). The top soil can be put to good use in garden beds.
That layer you want to compact so that it will be stable over time then you want to add the gravel for drainage.