A door fastening device for securement at a jamb (10) of the frame of said door (11), the device having a casing with a slot (43) in a recessed central part of a front face (38) thereof at which is disposed a faceplate (40) adjustably secured to the casing and having a slot (47) aligned with slot (43) to allow passage of a bolt (13) of a lock (12) or latch of the door. The bolt is arranged, when extended, to engage and move rearwardly in the casing a cam (24) secured to one side of a link (23) which is pivotally mounted at its respective ends to swivel bolts (21). The rearwards movement of the link (23) causes the bolts to extend through respective openings in the front of the casing to prevent opening of the door. The cam has a flat surface (29;31) and at 90° thereto an arcuate nose surface (36), the cam being adjustably rotatable to position the flat surface or the nose surface in line with the bolt. Since the nose surface is radially further from the cam rotation axis than the flat surface, the adjustment compensates for differing gaps between the door and frame by ensuring the bolt always moves the link fully to its position where it causes both swivel bolts fully to extend.