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While they may seem to be a form of overglorified Siege Workshop, this could hardly be considered close to the truth. While Forts do become the focal point of your gunpowder armies, they also perform a second task, and that is reinforcing your Borders — each new Fort constructed expands your Territory around it, thus making for a nice area-denial weapon to prevent enemies from successfully encroaching on your lands. They are only as strong as the units that fight them are weak, so remember to man them with units that can sally out should the foe bring siege weapons to bear.
Depending on your tech levels and your faction's tech tree, Forts can be upgraded to become more powerful, as will be discussed below.
Forts are available once Level 3 Military (Heraldry and code of honour) is researched, and depending on what you have researched, you can upgrade your forts to castles, and towers to keeps and bastions. Forts and Towers are upgraded to Castles and Keeps once Fortifications is researched, and can be upgraded again by Asian and Orthodox factions with Imperial Mandate at the Nobles' Court into Strongholds and Keeps, which are immensely strong.
Note, however, that despite being strong, Forts are vulnerable to siege weapons. The trebuchet, having the furthest reach in the entire game, can easily wreck anything with impunity regardless. Of lesser damage caused would be naval units, namely the Siege Galley; Carrack and Lanternas; a Castle with Military Engineering can match a Siege Galley in range; with Bombardment, Carracks are caught. Only a Stronghold with Bombardment can hope to hold off an attack by the Lanternas, and even so, several Lanternas will easily destroy a Stronghold through sheer determination and numbers.
The Stronghold is the Imperial Mandate-based upgrade of the Castle for China and Mongolia, and represents the powerful defences embodied in the modern form of the Great Wall towards the end of the Ming dynasty in China. As expected, it is stronger, tougher and more powerful than the average Castle, and can pose a challenge to most factions without access to the best bombardment units.
Without Fortification, Castles will well fall foul of enemy siege units. Fortification enhance a fort's tactical capabilities as well as its effects on your borders.
Strategy upgrades enhance the reliability of spies, and are also required for the creation of generals.