Imagine having a huuge building that takes an absurd amount of materials to build, something like 50 years of gathering so the difficulty to make it would be bigger than other buildings…maybe it could provide a small bonus but not a big one?
would like to hear your thoughts & opinions - its just an ideea, id love something like this to add more challenge to it 
love the game!
like one of the 7 wonders of the world?
can be a made up monument by the team 
that suits the game
In keeping with the period, Monumental Buildings could be:
Cathedral - a mega-Temple
Keep - a mega-Barracks (fortified)
Rathaus - a mega-Town Center
Trade Fair Grounds - a mega-Trading Post
Any or all of these could be ‘culminations’ of parts of the game, with massive extra benefits from the Monument related to the ‘lesser’ building it replaces.
And with perhaps a special Award for any gamer who manages to complete all of them on the same map.