If your office has a perimeter wall, then you can secure the same by following general Boundary Wall precautions as well using equipment for Perimeter Security
Keep minimum possible points of entry and exit. A single common point of entry and exit is the most secure but not necessarily the safest. For emergency like fire, there must be alternate exit point.
Doors not commonly in use should be kept locked from inside except those used as fire escapes. However, it should be ensured that passages to such locked doors are kept absolutely clear and free from any obstructions for use in emergencies. It is common experience that passages and staircases not in use are often utilized as storage spaces for old or spare materials or even litter. This poses grave danger in times of emergency.
All entry and exit points should be well lit or fitted with security lights or Intruder Alarm that are switched on automatically whenever someone enters the area.
Windows should have bars or a firm grill specially those on the ground floor or those that are accessible from outside. The bars or grill should not be screwed to the window frames but embedded into the wooden window frame or welded to the metallic window frame. There are many more precautions that are applicable as mentioned under Windows and Ventilators