only thing I have seen them do is arrest players, immediately for breaking law.
you break the law and they are there instantly to apprehend you.
they are The Flash!

a slight change to the arrest system would be nice.

an alarm is sounded.
3 seconds: city gates are alerted and start to check everyone entering and leaving.
10 seconds: search lights, exposing any stealthy characters as they search for character that matches description.
5 minutes: lights off, just gate check. and bounty placed for offender.

Guards attack any invasion critter.
Bounty System - placed on law breakers and invasion critters.

if you have a buff that would disturb the peace, guards stop you at the gate until you drop it.
if you have a warrant or bounty, they arrest you.