Mechanical defenses are very important in the event of threats arising from war, terrorism or civil unrest. As well as the stability of the foundations and load-bearing walls, the strength of a building's "external defenses" and consequently the level of protection they provide against attacks from outside is crucial. The objective is not only to prevent a targeted military attack, above all it is to protect the building from attacks by small bands of soldiers, terrorists, armed gangs or rioters. Since permanent mechanical reinforcement of the exterior of the building would be very costly, such measures could only be implemented as part of a large-scale renovation project. As a consequence, prevention should focus primarily on temporary mechanical strengthening of the weak points of the building's exterior, which in practice means doors and windows.