Abstract: A security system for roll-down loading doors, having: a substantially narrow profile housing adapted to being connected to a wall of a cargo carrying enclosure; an actuator having a key receptacle including a pivotable arm, for moving a latch structure between a locked position and an unlocked position; the latch structure being generally triangular with a top portion being hingably connected to the housing with a stationary pivot pin and the actuator being coupled by a linkage mechanism, the latch structure is adapted to be connected to a loading door of a cargo carrying enclosure when the latch structure is in the locked position and withdrawn from the door when the latch structure is in the unlocked position; and the linkage mechanism includes a distal section having a port and a proximal section having a second port, the distal section is coupled to a middle portion of the latch structure with a first pivot pin and the proximal section is coupled with the linkage mechanism with a second pivot pin, the li