Historic Cities have all their technologies at level 10 unlike level 1 to 10 NPC's. But quick well timed waves will break them very easy.
The Defending Royal
A number of different people have told me that the defending royal (King or Queen) within the Historic City have 10,000+ leadership, essentially infinite. In this case you must assume that all the defending troops fight at their maximum potential. The defending royal in historic city should have about 225 attack, with higher Intel and star set the higher the level of the Historic city.