The Port Bug has become a big issue in Shogun: Total War. When it was first discovered by players by accident it was considered a wonderful find. But since then it has been a subject of much debate.

What exactly is the Port Bug? The Port Bug is the ability for a player to move troops into an enemy's territory via a port. This is not mentioned anywhere in documentation. It is also never used by the Computer AI against the player. This is what has prompted it to be labeled "Bug".

How do you use it? If you have a port in one of your territories you can pick up one of your units from that territory and move it to one of your other territories that contains a port. This is how the ports are described functionality wise. Now you can't just pick up your units from your port territory and drop them on the enemy territory with a port. It is actually more in depth than that. You must first put a Special Unit in the enemy territory with the port. Once this is done you can move your units to the enemy's territory via his port.

The Special Units(Emissary, Shinobi, Ninja, Geisha, Priest) are in essence spying on the enemy's territory. This is how you find out that he has a port and are then able to exploit it. The spy you send does not have to walk across the land to get to that enemy port. By having the spy in your port territory and then picking it up and dropping it on an enemy port territory will allow the spy access even if you have never had a spy there before. But, when you pick up your special unit to move it the enemy port territories are not highlighted so you have no idea which ones contain ports. Its a hit or miss. Also, once you have discovered the enemy territory with a port you do not need to leave the Special Unit there, the territory has already been "discovered" so there is no need for continual spying. Even after an enemy territory is "discovered" to have a port, picking up a unit from your port does not highlight the enemy port you have access to. So you must keep a mental note as to where your accessible ports are inside the enemy territory.

With so much necessary to exploit this bug the arguement came up that it was not a bug but an undocumented feature. The developers put it in but didn't explain it and didn't have the AI use it. Either way some people use it as a viable tool to fight the Horde problem. Others refuse to use it because they believe it to be a bug and exploiting it is cheating. Since there has been no confirmation on this "Port Bug" feel free to do with it as you please.