From the West, Persian is difficult to invade, it has only been successfully done twice (Alexander the Great and the Ummayad caliphs a millennium later). From the east it was a different story, Mongols, Turks, Tajiks (who were the only actual Iranians to rule Iran until modern times) and Pashduns, would regularly invade, raping and pilliging and burning everything to the ground again and again until there was nothing left except crumbling adobe houses and that amazing national underground plumbing system.
The only group of foreigners to have the remotest claim tp "nativeness" were the The Samanids (819–999), who were the first in centuries to use Farsi as an official language in centuries and they hired the poet Ferdowsi to write the Persian national epic, "Shahnama: The Epic of the KIngs" which has been the core of Persian education ever since. They may have been forced to become Muslims, but Ins'shallah, they weren't going to give up their language like the Berbers or Syrians.
It's the poets who have kept the Persian people alive. Ferdowsi, Haifez, Omar Kayyam, and Rumi, all of whom lived at the end of the first millennium AD or beginning of the second, are treasured by citizens of the Islamic Republic far more than modern Brits do Keats or Shelly. Only William Shakespeare has such prestige.
In the city of Shiraz, people would go to the tomb of the poet Haifez in the evening and recite 800 year old poetry, before saluting the master with a Coke® (US trade with Iran is greater now than at any time since the Shah's fall). Could you imagine that with at the grave of Robert Frost? I don't think so.
In Isfahan, hundreds of miles to the north (eight hours by bus, and it feels it), people go under the pol-e Khadu bridge and sing tales from Ferdowsi (dig that crazy echo!) while later in the evening when it's actually cool, bring some of the family's more worn rugs and have a picnic on the banks of the Zayande river, where if it's possible to avoid notice by the morality police (which, from what I can tell, is the national sport), one can snuggle up with one's honey. The river at this time is filled with paddleboats. There is poetry is the vision.
The Mongols and Turks who ruled Persia for most of the second Millennium AD began to stop tearing down palaces of previous dynasties around the time of Henry VIII of England, and as these are primarily secular buildings, one can see a tradition of figurative fresco which looks like something out of Japan. However, the people in charge of restoration have done a horrible job in some places. In fact, except for a very few mosques, there's very little architecture that seems to be older than the 16th century, and what there is, is mostly adobe that doesn't look all that impressive from the outside. That is except for caravansaries, which are a cross between castles and hotels designed to protect merchants on the silk road.
It's at this point that a kind of weariness begins to creep in. We could feel the end of the trip creeping up on us, and not a moment too soon....