Landmarks can only be found on the Steam over Europe scenario. Each city has a unique landmark that cannot grow to a level higher than its associated city. It can only grow when it receives the necessary goods and funds, and only if its city has a higher level than itself.
Each level requires all goods that have been requested by the city, along with an increasing cash investment. Goods are only consumed when the landmark levels, not every 15 minutes as cities do. Landmarks can only grow to a maximum level of 15.
When a landmark grows to a new level, all the investors and suppliers will receive prestige based on their contribution. Only contributions made before the required amount is reached will count. Anything after this point will not earn prestige. As a landmark grows, regional points will be earned and the maximum level that the city can reach is also increased.