I do not know if other factions have access to these buildings, but at least as Rome there's one of the aqueduct line that gives extra agent levels to all agents recruited in the city. At the moment I have two of them in different regions, and those cities are where I get all my agents now. As it stands I'm getting rank 3 agents out of the gate, and in the campaign I'm closing in on being able to recruit rank 4 spies and dignitaries with rank 5 champions (I built the Colosseum).