Even though this is a bug, I haven't put it into the bugs section because it takes one page by itself (mostly due to the pictures). Shlonglor has discovered and analyzed it. The top reason why I've added this one to my strategy guide is to prevent people from making maps where this trick can be used (unless that's the purpose of the map).
Maybe you've wondered where your guys pop out after their training; this follows always the same pattern. I'll show this with the castle:
|16||15||14||13||First, the computer checks whether the top left corner is free. If that's the case, the peon pops out there; otherwise, the computer is continuing to check all other positions in a counterclockwise order. If all sixteen positions are used up, the peon will appear further away from the castle.|
The key is to build a foundry in a narrow river. If the foundry is finished, the peon pops out at the top left corner (if it's free), regardless of his old position! So you have to choose a position where the top left corner of the foundry will be adjacent to a land tile of the other side of the river.
Then, you build the foundry. Older versions of this trick included finishing it, but you don't have to finish building the foundry to make this trick work! This will save precious resources and speed the process up a lot.
Just cancel the construction of the building and whoops, there he is! On the other side of the island, you can build whatever you like: towers, barracks, even a town hall to start a new town!
With this trick, you can advance very quickly (only on maps that allow it, of course). Unless your opponent knows it too, he will be startled by the speed in which you advance to his city! Instead of having to build a shipyard and a foundry, you can put up a new town on an island for free. This is much faster than building a transport first. In fact, if someone doesn't know it, he will probably lose to this trick. The classical "foundry hop map" is High Seas Combat. Some players want to play that map all the time because they think they are the only ones who know the trick :-)
Of course you shouldn't do the hop with a foundry, but with a shipyard. The trick has become known under the name "foundry hop"; that's why I showed it with a foundry. However, it works equally well with a shipyard, and since you can build a shipyard before the foundry, doing this with the shipyard will speed up your expansion even more.
Hopping southward is tricky, but also possible; just make sure the top left position and all other following positions on your side are occupied by peons or water. Thus, you can jump into any direction you want.
Map creators should also be aware of that trick and make the rivers wide enough if they want to prevent players from using that trick. It makes some maps less playable, and if you want your water map to be used for naval battles, you had better make sure no one can hop over the rivers.