Guilds is very useful when resources are beginning to run out on the map, since it means the players receive more when they sell surplus resources (usually food). It is also cheaper to buy other resources.
The Saracens only pay 5% trading fee due to their civilization bonus, more than negating the fact that they cannot research Guilds.
The Forgotten Edit
The African Kingdoms Edit
"Political and economic power during the Dark Ages was held by barbarian chiefs. This power eventually shifted to kings, churchmen, and powerful nobles. Near the end of the Middle Ages the middle class of townspeople grew more important and took a share of this power. One innovation of the middle class was the creation of guilds. Guilds controlled a specific enterprise such as cloth trading, butchering, or iron making within a town or region. They set the prices and determined who would work in the business. Outsiders were blocked from working. In return for a monopoly, usually bought from the king or town, they provided a superior product. Controlling trades through guilds resulted in high standards of living for guild members, a quality product, and high prices."