Tripwire

The Reliefs of Trajan's Column by Conrad Cichorius. Plate number LVIII. (Photo credit: Wikipedia)

A tripwire is a small amount of troop that you send to support a city. If someone attacks the city, then you will receive notice about it being attacked.

Usually for a city on the same island, as your city, you send 1 swordsman as tripwire.

For cities on other islands, you can send a bireme or a transport ship with 1 swordsman. Note here is better to have 1 swordsman in the transport ship, because if the ship is empty, then it will die to light ship attacks. If there is a soldier inside, it will not die until the soldier dies, thus pure harbor attack won't kill it.

Advanced Tripwires:

The 3 unit trip: easy/more efficient

An "advanced tripwire", is when you send 3 troops to each city. Your troops are killed in proportion to the city owners troops:

1 troop killed approx 33% casualties no revolt

2 troops killed approx 66% casualties revolt possible but not probable send support

3 killed 100% casualties or they dodged and left your troops there send any support available asap.

The best (boat) trip: still easy/not as efficient/more accurate

The boat trip is where you send a single slow transport boat to trip an allied city. These work best because they do not die unless the city is cleared (all units have been destroyed).

Conclusion:

Always trip you're nearby allied cities, and don't be a buddy F'er.