Masterpieces are special buildings. They will be unlocked when the "unlock condition" is met.
A bonus will be provided if the "specified conditions" are met after construction. This is visible when placing/building the item. For instance if a Masterpiece has to be close to a river, that condtion will turn green when met. Or surrounded by forest (as a percentage) it shows 94%/100% in green, then that's sufficient to get the bonus.
Masterpieces can be built alone or in a complex.
Masterpiece can build up to three.