Cities: Skylines

The Space Elevator monument will be available to build once you've unlocked 6 unique buildings. Each unique building has requirements you must meet before they unlock. The buildings and requirements are as follows:

Statue of Wealth - City money: $500,000

Transport Tower - Public transport has 1,000 weekly passengers

Grand Mall - Construct an airport

Opera House - City extracts a total of 3,000 units of natural resources

Expo Center - 2,000 tourists visit the city

Stadium - City produces a total of 1,000 units of goods

Once those buildings have been placed in your city, the Space Elevator will become available. It costs $1,500,000 to build and $16,000 a week for upkeep. It will generate a massive influx of tourists into your city.