We have 3 Guard Cats plus a minimal (3 zone), monitored ($30/month) ADT Security System. Our house is a bit unique in that there is only 1 entrance in the front in full view of the street. Anyone coming in through a back, ground level window to get upstairs would have to pass through the front foyer where the motion detector is located. The 110Db horn there would take out their ear drums.
Not the greatest protection but it seems to have worked for over a decade. Perhaps it is the ADT 'yard signs' & 'window decals' that deter the amateur cretins...
TAM
