There are 8 game difficulty levels which lets you determine how challenging the AI opponents will be, and how it affect various game mechanics. 

Settler - Easiest level which is designed for beginners with significant player advantages and minimal AI bonuses.

Chieftain - Slightly more challenging than Settler but it is still beginner friendly. AI has a few bonuses while players still receive some advantages

Warlord - For more experienced novices with moderate challenges and AI bonuses. Players still receive a few advantages.

Prince - Standard difficulty where players and AI start on even terms. A balanced challenge for players.

King - A step up in difficulty. AI gains a few moderate bonuses and becomes more hostile. Players experience a slight reduction in their bonuses.

Emperor - Hard mode. Challenging gameplay with significant AI bonuses and aggressive tactics. Players encounter more tougher conditions.

Immortal - Very hard mode. AI has substantial bonuses. Players will be challenged with highly formidable opponents.

Deity - The hardest level. AI has the most significant advantages and aggressive strategies. Players will require advanced skills to overcome this challenge.