A system for the construction of life size play structures includes a multiplicity of inflatable building blocks, each being a member of a finite set of different multi-sided block types. Each such type is a cylinder having the cross section of a regular polyhedron, and the length of each side of each polyhedron is identical. The blocks are attached to each other by means of a rotating joint which aligns each contiguous side of each block with the contiguous side of the adjoining block, but which does not allow each such to translate relative to the adjoining block. The joints are constructed at fixed locations along the sides according to specific dimensional ratios. Stiffening means are used to maintain the dimensional integrity of each block, and to maintain the uniformity of the system from block to block.