Roundels can be deployed on roundels.
A roundel is a rounded artillery fortification. A roundel is not higher than adjacent walls.  Cannons can be deployed on artillery fortifications.  Roundels are the oldest artillery fortifications. Battery towers are an artillery fortification.
∀x (Roundel(x) → Rounded(x) ∧ ArtilleryFortification(x)) ∀x ∀y (Roundel(x) ∧ AdjacentWall(y) → ¬Higher(x, y)) ∀x (ArtilleryFortification(x) → DeployCannons(x)) ∀x ∀y (Roundel(x) ∧ ArtilleryFortification(x) ∧ ArtilleryFortification(y) → Older(x, y)) ∀x (BatteryTower(x) → ArtilleryFortification(x))