come looking for easy prey
Why would a Dragon go to a city for easy prey? They need a better reason to do that, like Smaug-Gold, revenge, baby dragons protection etc.
Anyway, in my conception they will target farms and isolated people in the camps and borders.
The most cheap and obvious protection for these people will be caves to hide and storage foods like that one:
And security by obscurity like don't use many torches, don't make too much noise and things that smell good like beef. Maybe even camouflage, even if in the medieval era they didn't know what is this!
They can make some way to notify the surrounding farms and cities know if a dragon is in the region with a system like used in LoTR, the fire signals
signal fire in the montain http://www.blackgate.com/wp-content/uploads/2014/01/The-Return-of-the-King-signal-fires.png