How to Rotate a object exactly 90°????

How to do a object like a camera rotate exactly 90°? I need to do a labyrinth like Window$ labyrinth screen saver and the camera need rotate 90° when reach a corner.

The best way would probably be using an IPO curve of rotation.

Save a keyframe of each of the 90 degree angles, with several frames inbetween for interpolation. Then simply use an IPO actuator to rotate the camera when needed.