↑Requires a Town Center to be built first, which means it does not apply to the first Town Center on Nomadic starts.
↑"Bonus damage" is incoming damage of all armor classes except for melee or pierce damage. It is also applied to Gaia damage, i.e. reduces the bonus damage of Wild Boars versus cavalry. "Land military units" excludes Monks. The bonus does not apply to hill bonus/cliff damage.
The bonus is applied to units and is thus preserved for other civilizations upon conversion. Conversely, if a Sicilian Monk converts a non-Sicilian unit, the latter will not receive the bonus damage reduction.
In practical cases, the bonus applies to (foot and mounted) archers, infantry, and cavalry. Note that while Petards are affected, normal Petards do not take any bonus damage to begin with. The bonus also applies to Flaming Camels and all Hand Cannoneers in scenario editor.