Tower guards attack players of an enemy kingdom automatically. They will not, however, automatically hunt other hostile creatures within their range, rather they will only do so when ordered to by a player. This can be done by saying either "help" or "guards!" in Local chat within range of a guard tower, after which the guards will attack your current target.
They will also attack [[KoS]]ed players, if player enters a deed that is protected by a [[Spirit Templar|spirit templar]] (this works on PVP and PVE servers).
Note that 2 Guards respond per turn but you can call more a 10 seconds later.