It's a really unsatisfying way to finish the map though, he's stuck on that tiny island with a huge army while you control 99% of the map. You've undoubtedly won at that point and just have to continually stockpile troops till you can overrun him.
Oh please, his capital has 10 gold/turn or so. It's not like he can add 10 tanks every turn. By the time you finish taking other capitals you'll have at least 150 gold/turn. Just crank out 10 balloons or bring those mainland armies back and you have more than enough to take on him.
Changing it to let the far side be on a separate tile wouldn't really change anything, except making it a player choice whether they just want to block the direct channel between the capitals or whether they want Karthan to be able to send troops elsewhere
And that "elsewhere" would be?.. Well I guess if you want more obstacles in your way you should have a choice. It would only mean that you need more Ironclads to shield your northern islands and territories, simply extending your blockade a bit further.
I also have a lurking suspicion that Karthan is hardwired to send transports only at your capital. Before I blockaded his island I never saw him send transports anywhere else. So having only immediate route blocked could easily mean he'll just send his transports round that tile.
