Monuments are large buildings which provide powerful bonuses to your city. However, they are expensive in terms of building materials and upkeep, and takes a considerable amount of time to construct. Building materials come in the form of resources and monument parts, which are either found on Treasure Hunts, built at the Provision House or bought at the Ruby Shop.
Construction of monuments take place over 2 or 3 phases, and each phase requires 8 resources. For construction to proceed, there must be at least 1 unit each of every required resource allocated to the monument. As long as any resource drops to 0, construction will be halted.
To complete a phase, Knowledge is also needed after construction is complete. Depending on the Monument, additional monument parts may also be needed.
The Library is the first Monument you'll build. Indeed, all other monuments require the Library to be built first.