What about if you created a model based on supplies. Therefore there would be flips that would be harder than others. For instance if the town had only minimal significance or had recently been captured then it's supplies shall be low and therefore will not be able to endure a blockade for too long (perhaps 1 or 2 hours) then more substantially fortified ports would have a larger amount of supplies and so would take (2-3 hours). This would then place the emphasis on resupplying the port. this then means that the port defence is not only focusing on one section of the player base but many. For example the traders to resupply the port, the pvp'ers to defeat or deter the blockading force and lower levelled players who would have smaller ships that could run small supplies or information (ports time until surrender or port battle) in and out of the port. With regards to the significance of the port, perhaps the lower ports provide 5 points towards victory and the stronger ports 10.