The Fort can be constructed on any of the following terrain and its (Hills) variant:
Attacking may be what wins games (especially for players who are trying for a Domination Victory), but defending well can make aggressive players think twice about launching an attack on you. This is when Forts come in handy - have your Military Engineers build them at choke points in your territory or in other strategic locations along your borders, then station some units there to fight off any invaders. Place Forts on hills for best results.
Poland can build Forts to annex the surrounding tiles, thus giving them a way to claim new territory and steal resources from opponents who settle nearby (and potentially "leapfrog" Forts to claim large swaths of territory without having to found cities there). The Poles should always keep a few Military Engineers around to use for this purpose.
Since the first ego-maniacal chieftain threw up a wall of dirt and proclaimed everything behind it his, soldiers have been building fortifications. Whether to pacify new territory, protect the borders, or insure that the ruled didn't get uppity, the military-minded have dug ditches and moats, planted stakes and spikes, and put up walls of all sorts. Some made such military engineering an art form, such as the Roman legions, which could march through the day and have a fortified camp in place by nightfall. In time, the appearance of gunpowder led to innovations in fortifications; those wooden and stone walls were simply no longer adequate. So, rather than build up, fortifications went down – trenches and bunkers, pillboxes and minefields, barbed wire and "defense in depth." But the intent remains the same, to protect the troops and keep the civilization safe from both without and within.