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Given enough time and opportunity and given the way in which traditional leaded lights are constructed it is plainly obvious that a thief can break through these old types of windows very easily indeed. That said, it is still worth fitting locks so that he is forced to enter this way and thereby increase the chances of detection. Instead of and or in addition to locks some people have installed specially made security grilles behind the windows where the steel neatly fits behind the leaded lights so not to spoil the visual effect. However, this can be a very expensive solution and not one that would appeal to everyone. As an alternative you could install secure secondary glazing, preferably of a type that has been certificated to BS 7950 Windows of enhanced security . If you do not want to apply a physical security solution or you are unable to because the building is listed then you should consider installing an intruder alarm. (Go to Secondary Glazing)
What is a secure window?
See Windows that should be locked for a definition