First of all, we strengthen the door frame. In its lateral bars, drill holes passing into a concrete or brick wall by at least 40-50 mm, as shown in Fig. 1. Drive steel pins 8-12 mm thick into these holes. You can also make a special part (Fig. 2), which is attached to the door frame with screws. Under the head of the part, make a chisel selection in the side beam of the box. From the outside, the door can be upholstered with roofing sheet so that it cannot be cut through the hole.
To protect the lock, make a U-shaped pad (Fig. 3) from a steel sheet 1.5–2.5 mm thick. Roofing iron up to 1 mm thick can also be used by adding bolt screeds through the hole in the door leaf. Under the lock, lay a steel plate fixed on the edge of the door with 100-120 mm screws.
Insert a piece of steel tube with a diameter of 10-15 mm into the door frame. Drive a steel pin from the rear edge of the door. Such device will not allow you to lift the door up and remove it from the hinges.
To prevent the door from being knocked out or squeezed out, place one of the locks half a meter higher than the others or directly above the floor - this arrangement of locks prevents the door from bending in one direction.
The reinforced door can be insulated and upholstered with decorative material or pasted over with a film, hiding the traces of "reinforcement-work".