you can get strong window guards that are attached to the window but the middle bars can swing out like a door leaving the whole area open for easy escape. The bars are locked in place with a padlock and you keep the key near bye nailed onto the wall in case of emergency for a quick exit
some examples http://www.securitywindowbars.com/Window-Security-Bars/window-security-bars-expandable-removable-swing.html
(but you can get even stronger and cheaper by locksmith probably)