The stronghold has a large river flowing through it. How about a new building, Harbor? Make it available at Stronghold 10 as an upgrade to a Farm at level 10, which produces 10% more food. Second harbor at Stronghold 15, and another at 20. Level up costs like Farm +1 level; so level 10 Farm becomes level 9 Harbor, which produces 110% of Farm level 9 and levels as Farm 10->11 transition.
When harbors are built, show boats on the river. Before that, maybe show a guy fishing off the walkways which are water adjacent.