Spiral, I don't think we actually see this, but I'd say yes, you'll have to go down at least some steps - otherwise you'd be on the same level with Mrs. Hudson's flat and that just wouldn't work.
And Mrs. Hudson's remark that she can't rent the flat out because it's a bit damp and that's the curse of basements wouldn't make any sense if it were on the same level as her own flat, I suppose.