In Call To Power 2, there is way to built Archer's tower (or with similar name). It can be build only once in city. I think, in Freeciv, there should be Archer's tower unit (not building like in Call To Power 2). Each Archer's Tower could have 1 archer as cargo. Archer's tower cannot move or attack, but archers in it could. It increase attack power of archers inside. Archer's tower should have 0 attack and 0 defense.
I think, you could instead add slots to city like in cargo, so ruleset moders could define, which building city could have and which units (also, how many).
Archer's tower
Re: Archer's tower
Ruleset options that would make it possible:
- One unit of a type per city (for the tower);
- Transporter influence on the cargo's attack (for the tower and the archer);
- If it is possible to unload the archer after the attack, then, actually, there is no "only one archer per turn gets the bonus" effect, so we should have a way to test if the attack happened ("UnitState", "Moved" or "MinMoveFrags" are not universally relevant).