Milestones

Milestones are a core gameplay mechanic, and the only way to unlock new features. Features unlocked through milestones include Zones, Transportation options, Amenities, budget/taxation options, and tools. Milestones can be triggered by a variety of factors, including population, time elapsed, the development of specific buildings, construction of specific amenities, and statistics like unemployment or city debt. Milestones are tracked per city, not globally.

If Tutorial Mode is enabled, all game features must be unlocked through milestones. If Tutorial Mode is off, certain features will be unlocked from the beginning of a new game, though the milestones that would unlock those features still appear.

Milestones
The "Size" requirement on some development conditions indicates that a building must have that length on at least one side. For example, the vanilla Supermarket design has dimensions of 2x3, fulfilling the Size 3 requirement of Grand Opening!