I've read a couple threads on this thing, but none of them have sufficently explained exactly how to blockade, and how blockading works. Does it require declaring war? If so, do you sit on the tiles to ensure the blockade works? Do blockade runners exist?
Thanks for your answers!
Note: I'm on Vanilla 1.74
Blockades in Vanilla and Warlords are much less useful than in BtS for a number of reasons.
Firstly, there are no Privateers, so you have to be at war with the civ you wish to blockade.
Secondly, blockades will not yield any profits for your treasury (as Elandal says, BtS's Privateers are the only units that can do this).
Thirdly, the area covered by a blockade is much smaller:
Fourthly, blockades will only cut off a city's trade routes if
all possible routes (including land routes) connecting that city to the rest of the world are blocked.
(I believe, although I'm waiting for someone to confirm or deny this, that in BtS a blockade can cut off all trade routes even if there are land routes available. This seems overpowered and unrealistic, but I can't find another explanation for the absence of any trade routes in some of the cities I've been blockading.
)
There is one very (very) minor advantage to blockades pre-BtS: they don't require a specific action. If your ship is in enemy waters, it will automatically blockade the tiles around it, whereas in BtS you have to click a button (and use up a movement point/all your movement points*) to start a blockade.
*a change in the new patch, apparently.
Overall, though, pre-BtS blockades are a minor feature, and will only be of much use at all in very specific situations. Occasionally, you can find an opportunity to starve a sea-dependent city by positioning a few ships in the right spots, or to cut a city off from vital resources. But these are quite rare. That said, if you're at war and your ships have nothing better to do, you might as well prevent your enemy from working a few coastal tiles in key locations.
But if you want to see some real blockading action, I'm afraid you'll need to invest in a copy of BtS.