In Tides of Conquest, you can expand your village into a city. It has various benefits, especially good for strategic positions, but comes with a price.  

Requirements

• Only on gameworlds that allow it (Tides of Conquest). 
• Town Hall level 20. 
• Empty settlement slot. This is the ability to settle/conquer a new village granted by Culture Points. So you can either have another village or convert the existing one into a city. 

Features 

You can upgrade your village to a city.

• The city gains 3 additional building slots.
• Resource fields can be upgraded to level 12 in non-capital cities.
• Walls can be upgraded with Watchtowers which give an additional bonus to the Rally Point option to see enemy unit types.
• Plus, Waterworks can only be built in Egyptian cities (including the capital if the capital is a city). 

Upgrading

It takes some time to upgrade a village to a city, depending on the gameworld speed.

• On x1 and x2 gameworlds it takes 24 hours.
• On x3 gameworlds it takes 12 hours.

The settlement slot required by the city is locked immediately at the start of the upgrade process.

Information about ongoing upgrade to city is visible in the player profile (on the village list) and on the map (in the tooltip when hovering mouse over the village).

If any other player conquers the city while the upgrade is in process, it is cancelled. However, if the owner of the village himself conquers it from another village, the upgrade is not stopped.

There is no other way to stop the upgrade.

Conquering

A city can be conquered in the same way as a regular village. After conquering, the city takes 2 settlement slots as usual. If you only had one free slot, the conquering still happens and the next slot will be used as soon as you gain it.