A cap block has opposed top and bottom major faces bounded by two opposed side faces and two opposed end faces. The bottom face has two grooves that extend lengthwise in a side-to-side direction of the cap block from one side face to the other side face. The two grooves are spaced apart from one another in an end-to-end direction of the cap block so that the cap block can be positioned atop and cap two back-to-back walls of wall blocks with each groove receiving therein protuberances that protrude upwardly from top faces of uppermost wall blocks of respective ones of the two walls. The cap block is manufactured by molding a dry-cast concrete mixture in a mold cavity having the shape of the cap block with a base of the mold cavity corresponding to one side face of the cap block.