By purchasing a legitimate license key from an official store and activating it through Steam, you support the developers (Colossal Order and Paradox Interactive) who made the game. You also gain access to official updates, Steam Workshop mods (safely), and online features. The few dollars you save by using a shady key generator or a fake ".txt" license file could cost you far more in stolen data, legal fees, or computer repairs. Always play it safe and buy your game keys from trusted sources.
Many websites claim to offer a “keygen” that generates unlimited Cities: Skylines keys. These are almost always trojans. Even if they produce a string of characters, the generated key will either be already used, blocked by the game’s DRM, or simply a random string of letters that fails activation. cities skylines license key.txt