City Tie Monitoring is a very old, and outdated way of monitoring a fire alarm system. Most systems have been upgraded to be monitored by some other means, but that doesn't mean that city tie is not still being used in some way, because the technology can still be used with the newer RF Radios as well as the IP/GSM systems.
So, how does city tie work? Great question! The system is very simple. In towns where the fire department still has their own system for it, there is a main Fire Alarm box that is outputting 24VDC on a supervised circuit. When a Fire Alarm system goes into alarm, this causes a short circuit, activating an alarm at the fire department, which will require them to dispatch out to the location.
Because this circuit is supervised, the FACP will be looking for a resistor, and go into trouble if the city tie circuit is open. It's important to note that when a panel is monitored by city tie, it will have separate city tie circuits for alarm, trouble, and supervisory.