One advantage of the Notre Dame is that the 10% savings allow you to forgo fully upgrading the 3rd market and mill, which saves villager upgrade time at the beginning of an age (when trying to pump out costly troop upgrades from the blacksmith/armory). Furthermore, it makes resource protection much easier by permitting you to essentially sacrifice the lowest level market and/or mill. Colosseum becomes less useful as a defensive structure in later ages due to prevalence of range-based armies and biplane/triplane support. Nevertheless, 'Strength of Gladiator' is a very useful tactic if employed correctly. However, I must admit that the only Classical wonder that annoys me is Forbidden City.