After all, plain glass is easily breakable. Consider replacing ordinary or toughened glass with laminated glass, two pieces of glass bonded together with a sheet of laminate. This is far more difficult to break through as it will not shatter and will therefore delay any attempt at forced entry, a valuable deterrent. Alternatively, you can place a plastic glazing film over the glazed area to prevent the glass from shattering, though the clarity of the glass will be reduced. When replacing glass in wooden windows use a glazing mastic to bond it to the frame, as it is far tougher than putty.
Leaded windows are particularly susceptible in that they offer little resistance to attack.
Lead is a soft material with poor tensile strength. To protect them you can install secondary glazing, or, alternatively, fit metal grilles or bars. Such measures can detract from the overall look of your windows, but it is important to be aware of their vulnerability.