Maybe it should require desert or something?
Sounds legit.
Yet another suggestion:
Wonders that give free buildings seem too powerful, especially modern wonders because a) they are not religion-specific and so the tech leader can easily grab all of them b) the free buildings themselves are high cost buildings.
These wonders include the the Krak de Chevaliers (castle), the Versailles (estate), Abbey Mills (sewer), Westminster (courthouse), Chateau Frontenac (Raliway Stn), and Brooklyn Bridge (Levee).
Maybe these wonders could be adjusted so that
a) they give production boosts to the buildings instead of granting free ones.
Brooklyn Bridge would still be powerful if its effect was nerfed to 'double production for levees'.
b) they give bonuses to the buildings. In civ2, the Oracle doubled the effect of temples. For example Westminster could grant free statemen to courthouses.
This way the wonder would actually synergize with the Constitution civic and not diminish its effect.
The Abbey Mills could grant additional health from sewers.
c) at least require the building they are granting.
It is kind of strange to see the Krak des Chevaliers being built in a city without a castle, or the Chateau Frontenac built in a city without a railway station.
(Probably this should be the case for some other wonders as well,
like the Bell Rock Lighthouse should require a lighthouse and the Flavian Ampitheater should require an arena)