For what it's worth, I'd never recommend building an early Slinger. It's much safer to get an extra Warrior or two out first, then build a Slinger to get the Archery boost once you already have your city well defended. Slingers just don't cut it for early game protection.
Also, barbarians won't take your capital, although that's cold comfort if they bottle you up before you can get a settler out to found your second city.
Speaking of which, the best defence for your capital is a second city 3 hexes away. Which pushes you into a relatively rigid Warrior-Warrior-Builder-Settler build order that leaves a Scout as a luxury. And back to my prior post, that seems contrary, to me, to the whole idea behind Prince- or lower level games, which shouldn't offer that much danger early on. I think it's perfectly fine for Immortal+ games, but they're not really much more difficult (adds the risk of early Warrior rush, but you handle that the same way you handle early Barbarians, so what's changing for the most part is the % chance that you lose because you didn't go W-W-B-S to start).