Age of Empires II: Definitive Edition (PC) - Achievement - Pitched Battle

Successfully defend Cuzco without constructing any new walls or gates in the second Pachacuti mission "The Field of Blood".

This achievement is pretty self-explanatory - Simply don't build any walls or gates. Considering this mission revolves around the protection of a wonder, this achievement condition could be considered a hindrance ... but nobody said anything about not building castles. Simply make sure the wonder is protected by Castles or units from all flanks and this shouldn't be a problem.