Vanguards' only ranged common. Early and mid-game you should try to always have one archer around if possible. They are very strong early game, easily having 3 to 5 attack strength with minimal cost, and they are very hard to trade with because of their high life. Unlike the rest of Sera’s plan, Citadel Archers do not require protection, as they have a great life to magic ratio. If you want to poke things, attack things or threaten carelessly, Citadel Archers are your tool.
As the late game starts to kick in however they lose a lot of their value and effectiveness. Part of the reason late game Vanguards needs to become more aggressive to delve into melee exchanges is because of this unit's dwindling efficiency.