A closure includes a panel closing against a frame. A set of abutment surfaces for supporting the panel against applied forces is completed by a displaceable abutment block which selectively assumes an engaged state disposed between an abutment surface of the panel and an abutment surface of a strike jamb. The geometry of engagement is such that forces acting on the abutment block opposing opening of the panel are primarily compressive forces. Certain implementations additionally, or alternatively, employ abutment blocks for locking along the lintel or the threshold of the opening.