Disgustipated
Deity
In addition to tactics: if you see more than one enemy unit near one of your cities in the earlier game, prepare for war. That often gives you 3-5 extra turns.
This is very important. I notice they are stationary/fortified maybe and if you see them not move from your border several turns, time to switch from whatever you were building, and switch to military units if you don't have some. Move your exploring units to the threatened city.
Yeah, one Slinger probably doesn't cut it, a lot of the time.
Slingers suck for everything. I NEVER rely on these for defense. Their only use is to try to get the archery Eureka, that's it. I don't even like to build more than 1 slinger (for inside the city), warriors are more versatile.