Enclosed structures, such as guard towers, are provided which are designed to be easily transportable, constructed easily onsite and erected in a manner which produces an exceptionally blast resistant structure that is also advantageously designed for positioning within a perimeter wall. In particular, in some embodiments, the guard towers have a pentagon shape and are comprised of a plurality of composite panels. Each panel is able to be constructed onsite. The panels typically have a rectangular shape with at least one half of the panel constructed as a solid composite of concrete and metal. Once the panels are constructed, the panels are erected and arranged in a pentagon shape. The panels are bolted together with the use of vertical face connections. These connections provide better structural integrity, retained over time, particularly after subjection to a blast. In addition, the pentagon shape provides improved safety when positioned along a perimeter wall.