Advance
Naval Battery
The natural evolution of defensive warfare has culminated in the development of intricate coastal defenses, ready to repel even the heaviest of hostile invasions.
Advance tree
The game files don’t say where this goes: it has no requires, so the game places it in the 
Reformation tree itself.
Definitionin_game/common/advances/1_building_unlocks.txt:894
897ai_weight = {
898if = {
899limit = {
900has_ports = yes
}
902add = {
903value = 50
904multiply = num_of_ports
905divide = num_locations
}
}
}