If a thief is determined to get into your home, he probably will, if he is not disturbed and has ample time. Any precautions you take are directed at making his task difficult so that he does not persist.
A mortise lock is fitted into the thickness of the door and is difficult to force. The notches on the key engage with levers. The more there are of these, the more difficult it is to pick the lock. Single or double levered locks are only intended for inside doors. If the lock assembly is withdrawn after removing the retaining screw, screws removed from the faceplate of the lock allow it to be removed and opened. It can then be cleaned and lubricated, preferably with graphite powder, but a light oil can be used sparingly. Too much will attract dust and dirt.
If the lock is difficult to work, a spring in a lever may have broken. A locksmith can provide a new one. If you want to change keys, different levers can be fitted to the lock.
A rim lock fits on the inside of the door and externally there is a cylinder to take the key. Although this type of lock is difficult to pick and the variety of keys is considerable, there are other ways of forcing it. A piece of flexible plastic, like a credit card, can be pushed through a slight gap to force the bolt back. A snugly fitting door with a good rebate on the frame will reduce this risk.
A catch on the inside of the door will prevent forcing in this way, but if there is a glass panel that could be broken and the lock reached, it is better to have a night latch, where the key can be used inside and removed in a similar way to a mortise lock. Open the lock and lubricate it occasionally. The cylinder can be lubricated by smearing the key with graphite or oil and working it in the lock.
For double security there can be a rim lock and a mortise lock on the same door. Let the mortise lock take the main load near the middle of the door and position the rim lock higher. Fitting it involves the drilling of one hole for the cylinder (usually 32 mm). This is better than having bolts as they cannot be used when you leave the house unattended.
A chain that allows a door to be opened a little to see who has called before opening fully, provides protection against unwanted callers who may try to enter. The chain hangs from a fitting on the door frame and can be hooked into a slotted piece on the door, when the door is closed. It cannot be released when the door is partly open. You have to shut the door again to release it.
Fit the piece from which the chain hangs to the door post with screws at a convenient height. The chain hook can usually be left hanging in this when not wanted. Opposite this hold the slotted piece into place on the door edge and mark its position. It has a part that turns over the door edge. If there is much clearance to the door this may not have to be let in, but otherwise cut a shallow notch for the plate, like that for the edge of a rim lock. Screw the slotted plate to the door and test the action.
Sash windows, where two parts slide vertically and have a catch between them when closed, are particularly vulnerable to potential burglars. The small catch between the two parts can be pushed open easily. There are catches where a screw or spring prevents the swinging arm being moved until it is released inside.
This catch holds the window closed, but if they are to be left slightly open for ventilation there are bolts which can limit the movement of the windows. A plate with a threaded hole is screwed to the side of the upper sash at the point where the lower sash is to come when either is opened. This could be about 100 mm (4 in) without a would-be intruder being able to get his hand in to release anything. A screwed plug is turned with a key
(which can be removed) into the plate and its projection prevents the overlapping sashes being moved further.
Sash windows are counterweighted by lead weights on cords in boxes at the sides or with helical springs in modern factory produced windows. There are removable strips down the sides called beads, which enable the weights and cords to be reached for repair. The beads act as guides for the windows and are often very lightly attached, so that if one is removed the window can swing sideways.
Casement windows, swinging on vertical hinges, are not such a problem. Make sure the catch on the opposite side to the hinges can go fully home. If it will enter only part way, it may be shaken free. Similarly, if the stay at the bottom drops fully on to its pegs, it is unlikely to be moved. One of the pegs could be threaded and a nut could be put on it.
See that casement and other swinging windows close tightly, not only against their stops, but into the recesses. A gap does not need to be very wide for a wire to be pushed through and manipulated to move a catch. If a house is to be left unoccupied for some time, screws can be driven to prevent catches being moved until the screws are withdrawn.
Double glazing also provides security. Even if the outer windows can be forced, the intruder is unlikely to try to move the inner glazing panels as well. Lock manufacturers offer window locks which operate with a key.
If you wish to check anyone at the street door there are door answering telephones and even one that gives you a picture of the caller. A simpler (and cheaper) device can be fitted at head level through a hole in the door. By the use of lenses you can have a broad view of the outside of the door, but the visitor cannot look in. Only one hole has to be drilled to fit this.
If you want more sophisticated protection there are systems which sense when windows or doors are opened, carpets are trodden on and things moved. They set off alarms which should scare off an intruder. The better versions of these have to be installed professionally. Partial protection with a simpler installation may give a false sense of security.