There would have to be physical barriers around boulder, its what made it safe in the first place. Not around the whole city necesarily but around whatever part they secured and is holding that population. I very much doubt that any z's could just wander in and I'm sure sneaking in would be difficult as well. I wouldn't doubt if there are several other electronic and physical means of monitoring and securing the area, such as cameras, trip wires, claymores, sensors etc etc...I don't think anything can easily get in no matter how lax the guards might be. Redundancies should be in place to not allow any one person or action to make it exposed. No way Boulder goes down unless multiple people f up huge.