I've greatly underestimated Saka Horse Archers. Like, BIG underestimation.
I first thought they were Slingers in a horse. Wouldn't be that bad, but not stellar though. I just figured their so limited range would be too big a hindrance.
Then I used them, and boy, they pack a punch. They're more like Horsemen with a sling (maybe catapult given how much damage those "slings" deal). Surround a city with them and they can bring down Ancient Walls in 2 turns.
Pair them with all of Tomyris' abilities (double light cavalry production, extra damage when enemy is wounded, heal on kills), the card for 50% production for cavalry units and the fact they don't need horses because they're a unique, and you get a nigh-unstoppable early army. Forget the 10-cities-by-turn-100 milestone, depending on the map size, dificulty setting and number of players, you may be looking at 12 cities and up.