If I were "them"...
The unmarked cars or aviation would come into the area for installing electronic surveillance on wireless devices at the minimum.

They would arrive, linger for a minute or two once in range of devices with a femtocell, and then leave.
There is simple and cheap technology for doing these sorts of things available to all law enforcement.
http://thehackernews.com/2016/01/drtbox-cellphone-interception.html

There is a fun project underway for detection/countermeasures against these that is quite useful for protecting a geography you control.  It already does most of what you might want and is not so expensive.
http://hackaday.com/2016/08/09/how-to-detect-and-find-rogue-cell-towers/
Much cheaper than castle walls.

If a castle wanted a more active defense, HERF devices are very effective against most vehicles.  It would have to be abandoned.  If flying it would likely crash and be messy.
https://www.theengineer.co.uk/issues/october-digital-edition-2/radio-beam-device-can-disable-car-and-boat-engines-from-50m/

The detection of these sort of active defenses is such that it would benefit from underground wiring so the heat from the wire would not draw a line to all of them from higher altitude infrared scanning.  A good amount of capacitance at the device location is useful, so that it would still function if the wires were cut.  As with any technology, maintenance and testing is the ongoing chore.
Just some fun things to think about.