Remember to remove keys from locked windows and to keep them out of sight in a safe place. Louvre windows are especially vulnerable because the slats can be removed easily from the frame. Glue the slats in place with an epoxy resin, and fit a special louvre lock. Better still replace them with fixed glass. Casement lock s make it impossible to open windows without the correct key.
Fanlight locks have a metal bolt to secure the metal arm used to open and close the window.
The lock shown here locks the two windows together. A more discreet version is embedded into the wooden frame. Or there are devises to stop the window opening beyond a certain limit. Some exterior lights have an infra-red sensor that switches the light on for a few moments when it detects something in its range. Sensors can be bought separately to convert an existing outdoor light into a security one.