WhittyMike
Chieftain
Tired of all those foreign settlers crossing your borders without permission to create barely-viable villages in the gaps left between your own cities? 
Can't afford to build enough military units, even Warriors, to totally block off that border?
Looking for some way to stop this infestation?
Well, here's a solution for Expansionist civilizations: Build many cheap Scout units and use them as sentries and border patrols.
Later-stage civilizations can do the same with Explorers.
Granted, these cheap units won't prevent an actual military attack by another Civ that's willing to go to war with you. But they will block an invasion of foreign settlers, at a lower cost than military units.
Whim

Can't afford to build enough military units, even Warriors, to totally block off that border?

Looking for some way to stop this infestation?

Well, here's a solution for Expansionist civilizations: Build many cheap Scout units and use them as sentries and border patrols.
Later-stage civilizations can do the same with Explorers.
Granted, these cheap units won't prevent an actual military attack by another Civ that's willing to go to war with you. But they will block an invasion of foreign settlers, at a lower cost than military units.

Whim