A strategy where you spend almost nothing on military in the Feudal Age and go straight to the Castle Age, often called FC. You build the two Feudal Age buildings you need to advance (usually a Blacksmith and a Market, or a Stable for Knights) and click up as soon as you have 800 food and 200 gold.
A Fast Castle is usually followed by Knights, Crossbowmen, Monks or extra Town Centers (a boom). It is most common on closed maps like Arena, where walls protect you. On open maps it is open to Feudal Age aggression, so players often add a few military units or walls first (see Drush FC).