A system for building large-scale play structures, the system comprising: a plurality of panels (500A, 500B, 500F, 500G) having a thickness, a planar-quadrangle shape including a length, a width, a first planar surface, a second planar surface opposite the first, four chamfered edges with a set of holes located along each edge, the holes provided as through holes extending from the first planar surface to the second planar surface and located at a fixed interval relative to one another, the fixed interval providing a unit of length, the plurality of panels (500A, 500B, 500F, 500G) including four panels including a first panel, a second pane1, a third panel and a fourth panel; a plurality of fixed-location fastening elements located within the holes; and a plurality of movable fastening elements each including a hand-grip to allow the respective movable fastening element to be moved between an unsecured position and a secured position without the use of tools when received by any one of the plurality of fixed-location fastening elements, wherein each set of holes includes at least a first hole located at a first end of the respective set of holes, a second hole located at a second end of the respective set of holes and at least one intermediate hole located between the first hole and the second hole along an edge of a central region of the respective panel in which the set of holes is located, wherein the first panel, the second panel, the third panel and the fourth panel are configured to assemble at right angles such that the four panels are secured together to form a hollow quadrangle-shaped structure when the assembled-panels are viewed edgewise, wherein a distance measured from an outside plane formed by the first panel to an outside plane formed by the second panel provides a first dimension, the second panel located opposite the first panel in the structure, wherein a distance measured from an outside plane formed by the third panel to an outside plane formed by the fourth panel provides a second dimension, the third panel located opposite the fourth panel in the structure, and wherein the first dimension equals an integer multiple of the unit of length plus the thickness, and wherein the second dimension equals an integer multiple of the unit of length plus the thickness. a plurality of panels (500A, 500B, 500F, 500G) having a thickness, a planar-quadrangle shape including a length, a width, a first planar surface, a second planar surface opposite the first, four chamfered edges with a set of holes located along each edge, the holes provided as through holes extending from the first planar surface to the second planar surface and located at a fixed interval relative to one another, the fixed interval providing a unit of length, the plurality of panels (500A, 500B, 500F, 500G) including four panels including a first panel, a second pane1, a third panel and a fourth panel; a plurality of fixed-location fastening elements located within the holes; and a plurality of movable fastening elements each including a hand-grip to allow the respective movable fastening element to be moved between an unsecured position and a secured position without the use of tools when received by any one of the plurality of fixed-location fastening elements, wherein each set of holes includes at least a first hole located at a first end of the respective set of holes, a second hole located at a second end of the respective set of holes and at least one intermediate hole located between the first hole and the second hole along an edge of a central region of the respective panel in which the set of holes is located, wherein the first panel, the second panel, the third panel and the fourth panel are configured to assemble at right angles such that the four panels are secured together to form a hollow quadrangle-shaped structure when the assembled-panels are viewed edgewise, wherein a distance measured from an outside plane formed by the first panel to an outside plane formed by the second panel provides a first dimension, the second panel located opposite the first panel in the structure, wherein a distance measured from an outside plane formed by the third panel to an outside plane formed by the fourth panel provides a second dimension, the third panel located opposite the fourth panel in the structure, and wherein the first dimension equals an integer multiple of the unit of length plus the thickness, and wherein the second dimension equals an integer multiple of the unit of length plus the thickness.