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.