How to secure apartment windows with external fire esscape
Alarm
You could pick up a magnetic window/door alarm (some more expensive models may also have glass break sensors).

If the window is opened, the alarm will sound.
Secure the window
You could purchase, or make a &quot;security bar&quot; similar to the ones used for sliding doors.

This will make it more difficult to open the window, though would not protect against breaking the glass. If you make/purchase an adjustable version, you would even be able to leave the window open slightly and still be protected (as long as it's not open enough for the intruder to slip in).
Protecting the glass
There are many companies that make Security Window Films, that can be applied to glass to make it &quot;unbreakable&quot;.
Depending on your budget for this project, a combination of these products would be the best solution.