Currently, if I understand correctly, if you want to move ships from one side to another you have 4 options: 1) moving them all away around the continent to get to the other side 2) move through a fort that has coast tiles on either sides through friendly territory 3) move through a citadel that has coast tiles on either sides through friendly territory 4) make a series of forts and citadels and connect the two coasts I was thinking maybe there is another solution. Humans have been building canals since 4000 BC. What if around the industrial or modern era; 1) a special building has to be built in the city controlling the territory (maybe a dam?). Then workers gain the ability to add rivers to tiles. Special rules can be set up so the rivers can't be spammed (so for example to make your city harder to capture) 2) friendly ships gain the ability to move over rivers, while becoming civilian units while they move. That way you can connect the two coasts. As for realism, well, I got nothing.