Celtic archers, while not great or exceptional, are good. They are well trained, disciplined, and can put a fair deal of range behind their attacks.
Celtic archers, Sotaroas (Sow-tah-rows, "Bow Soldiers"), while not great or exceptional, are good. They are well trained, disciplined, and can put a fair deal of range behind their attacks. While their arrows aren't too notable, their ability to swiftly pepper an enemy with arrows is always of use. They are not really meant to defeat an enemy force, or even truly damage one, so much as they are meant to irritate and draw attention away from a main force, so it can position itself for flanking. As such, they are good runners, who fire a few volleys as a distraction, and then withdraw to a safe position to hide. If caught in a melee, they would be slaughtered for sure.
Historically, archers in Celtic culture had little variety, with only a few truly notable types of archers. They were irritative forces, and intended to soften large enemy forces, but rarely meant to be truly effective in the manner of eastern archers. However, if positioned properly, or experienced enough, they could be truly devastating. Their skill was good, they could fire many volleys swiftly. The Celtic concept of archery was more often to simply blacken the sky with arrows, than it was to fire few, effective volleys. However, their arrows were well made, though not great, and would be capable of puncturing lighter armors, and were quite deadly in their own right. All the same, these men were better off as hunters than field warriors.
Despite being the second weakest archers in the game (the worst being Toxotai), Sotaroas have their uses. Their low cost makes them ideal garrison forces, especially for inland provinces that do not need big garrisons. They are also useful in sieges, where they can torch enemy siege equipment, or for besieging towns with wooden walls.
With a low arrow supply, poor range, a small unit size, as well as practically no armour, they are not recommended for missile duels with other archers, but rather for provoking the enemy and decimating skirmishers or falxmen at range. You can use them against slingers, but don't expect great results (except against Accensi who have even shorter range).
Perhaps the greatest asset of this unit is its secondary weapon, a spear, which is much better for self-defence than the secondary weapon many other light archers carry. While this does not make them line infantry, it is certainly of use against light cavalry that might try to pick off archers in melee - which might be a bad mistake when attacking archer-spearmen such as these.
In Britain, most of Gaul, and Iberia (there as mercenaries only), Sotaroas are the only available archers, which means one has to make do with them. Incidentally, these regions sport many unarmoured enemies, therefore it is always wise to field Sotaroas on your campaigns in central and western Europe.