Wow, I've never seen Barb horse archers. Archers, axes, swords, longbows ... but never any horses, except maybe a rare chariot.
Yes, do not underestimate the power of sentries. Ever since I've started placing two sentries per city just on the edge of my culture boundaries, in woods or a wooded hill, I've done well. Before that, barbs meant a huge disadvantage. Does dual duty of reducing places for barbs to appear (they can only generate or found cities in the fog of war), but they tend to attack the unit nearest them, and so will suicide against a fortified archer on a wooded hill.