A metropolis will make your empire more powerful in general and it will lessen the management and organisation that your kingdom needs.  A fully upgraded metropolis will have the following benefits:

• Embassy 
  • higher silver and copper capacity (20.000.000)
  • better exchange rates for silver (1:35)
  • shorter distance to the market from the keep (10 fields)
• Military Academy 
  • better unit attack power (35%; +5% compared to a city)
  • better unit speed (30%; +10% compared to a city)
• Metropolis Wall
  • better base defense (448.000)
  • higher defence bonus (175%; +25% compared to a city)
• Port quarter
  • better capacity bonus (40%;  +20% compared to a city)
  • New unit: Hulk
• Entertainment quarter
  • better mission bonus (40%;  +20% compared to a city)
  • new missions 
• Academy 
  • more resource production via knowledges (10%)
  • more unit speed via knowledges (10%)
  • new knowledges
• Residential Quarter
  • more possible subejcts (2.000.000)
• Production buildings 
  • a significant better resource production for wood, stone and ore 
• Storage buildings 
  • more sotrage capacoty for wood, stone and ore (4.500.000)

But beware: The metropolis is not as powerful as the individual cities were just after it has been created. It must be upgraded in order to achieve its true potential.

Attention: All active banners will be taken down when the cities are merged into a metropolis.