Development is the measurement of technological advancement and general infrastructure in a County. Development directly increases taxes and levies you get out of the holdings, and it also unlocks some other special options. Development increases very slowly across the duration of the game, and radiates outwards from high-development Counties to those nearby. For example, Constantinople (aka the City of the World’s Desire), starts with a very high Development level. This will slowly spread outwards, reaching the most remote areas much slower than their Greek heartland. Naturally, there are other ways to increase your development, such as through the Steward’s ‘Increase Development’ task, although this is a fairly slow process, and usually only worth doing in certain Counties. Having terrain such as Farmland or Floodplains in your Counties make them ideal candidates for development, and when they have gotten some levels of development you can just sit back and enjoy, as it slowly spreads throughout the rest of your realm!