Can topsoil be used as a base for a terrace?

I thought the same, just use the topsoil as a base. That didn’t work and the weeds grew like crazy here. So the slabs were taken off again, the topsoil removed, and then gravel was added. That holds better now and the weeds are no longer visible.

That is exactly the right solution. That was also my plan, and I couldn't manage it. As an addition, I applied basalt 0/32.

Spring is coming again soon.