A foldable structure includes: an upper assembly forming an upper portion of the structure; a base assembly forming a lower portion of the structure; a centrally-located hub assembly operatively connected to the base assembly; and a post extending from each corner of the upper assembly to a corner of the base assembly. At least one rotatable wheel assembly is provided at each corner of the lower portion of the structure extending from the structure to engage a surface. Each wheel assembly includes at least one support surface adapted for supporting and guiding the wheel on a surface and at least one groove provided on opposing sides of the support surface.