Let There Be Light
What do you do if there are windows at the back of your property — which must apply to just about everybody except non-ground-floor flat dwellers?
You can consider grilles. I don’t like the huge concertina grilles very much. They look overpowering even when drawn back; and they can trap fingers. Strap grilles are quite good; they’re fairly unobtrusive.
For a general increase in security at the rear once you fitted good locks, the next thing to sort out is light. Obviously there are daytime break-ins so it’s not an absolute. But it’s a good deterrent when a thief raising its ugly head over the fence/wall/trellis suddenly activates a nice powerful light.
But do have them infra-red activated, do adjust them for sensitivity and for daylight level, do put them up high and do get them with a metal grille over the front.