Milestones are achievements that unlock new, important for the city and the mayor buildings and political options. They unlock by reaching a specific amount of citizens.

Amount of citizens required to unlock a milestone varies and depends on map. If map is easy, then you need more of them, if it's hard then you need less.

When a milestone is reached you are awarded with few dozens of thousand credits. New structures and political options are unlocked, such as the ability to create districts of a specific type or new city regulations. You will also have the possibility to place new unique structures.

You should be very focused on getting a pretty quick growth rate. It will allow you to receive many important structures before their worse substitutes will stop doing their function efficiently.

Elements that are worth quick unlocking are:

• Highways - they allow to move citizens very quickly
• Advanced wind turbine - despite what is usually said about them, they are efficient and cheap. And they don't pollute the environment.
• Incineration plant - creates electricity from garbage. Important building as landfills tends to get full quickly...
• Crematorium - same as Incineration plans, as Cemeteries don't have infinite space...
• Solar power - cheaper than Wind turbine.
• Water pumping station - reduces water pollution by 85%. Additionally, it is 25% more efficient than sewer pipe.

All the other needed buildings will unlock on the way. All those structures should be unlocked when city reach 20-25 thousand of citizens.

Further milestones unlock Ports and Airports.