Suggestions on "smart" enemy.

hello guys

I have a character that is moved by a joystick. Our protagonist has to move in a path. At the same time I have a monster that has to chase him. To do so, of course, the monster has to follow the path and can not get out off the road.
The monster obviously has to change direction every time the protagonist changes position in the map.

the map is uploaded on the attachment

if you can’t see it just cick here

http://imgur.com/0Dmv6LQ

the brown part is one in which you can walk. The monster has to walk inside this part.

I’ve already insert the waypoints somewhere. But I don’t know how to proceed to make monster move only in inside the walkable part of the map

Have you got any suggestions??