Cities Skylines features a milestone progression system. Each time you reach a certain population milestone additional gameplay and related buildings, overlay and settings become available. Checkout this guide for an overview table of all milestones! I will also do my best to pass judgement on all unlocks.
Cities Skylines is an interesting game that is pretty accessible compared to some of its competitors. However there are a few things that might not be immediately clear or that can even allow you city to perish in a few minutes.