I guess i would like to be able to look back during the event to see what part of this logic didn't work. Then I started thinking, can I reference Node custom properties even though I have the "Type of Property to Monitor" set to Interface?
It should work as long as you don't change the "Type of Property to Monitor". Leaving it on Interface and then selecting your trigger condition -> Network Nodes -> Custom Properties -> City should work just fine. Just make sure you don't change "Type of Interface to Monitor" back and forth within an alert, because then it won't work. If I have an alert that doesn't fire when everything looks right I usually just blow it away and create a new one. I don't know of any way to step through the alerting process, but that would definitely be helpful.