The question
At the time of the European invasions after 1492, the most technologically advanced cultures in the Americas had progressed to about the same stage as the Bronze Age in the old world. However, among other significant differences, the cities in Pre-Columbian American cultures generally lacked city walls while city walls were the defining feature of Old World cities at this technological stage. Why is this?
The states
The relevant Pre-Columbian American cultures are Inca, Maya, Muisca, Mound-builder, Aztec and related cultures that each possessed (many or almost all of:) writing, large-scale empires, extensive agriculture, street and waterway infrastructure, trade networks, bronze, wide range melee and projectile weapons, huge cities, large scale military campaigns and public works.
Comparable cultures in the old world would have been the old and middle kingdom in Egypt, the late Sumerian, Akkadian, early Babylonian empires in Mesopotamia, the Indus Valley civilization in India, the Minoan and Mykenean civilizations in Greece, the Hittites in Anatolia, as well as China up to the Zhou dynasty.
The walls
All (?) the mentioned Old World cultures relied heavily on city walls as defensive features. This is reflected in the histories and myths of the time. Pre-Columbian America lacks this feature in spite of ample conflict including conquest and sacking of cities. What little seems to have been written about this (see Gat 2002, especially p.9 and following) hypothesizes that city walls only develop slowly over time and would not be expected to be present in the early Bronze age. They are, so the article, mainly found in later stages, around well-planned highly populated late urban centres. In the light of the facts that some pre-Columbian cultures had an urban history of more than 1000 years (in the area) and were both well-planned and highly populated (Tenochtitlan being among the largest cities on earth at the time), this does not appear entirely convincing.
It seems that in many Pre-Columbian conflicts, the last stand of the defending side did not take place around the city (on the city wall) but on the city's pyramid-temples (providing high ground and naturally defendable positions). See this account of the conquest of Tlatelolco by the Tenochca's (freely available here); the defeated king/tlatoani Moquihuix took his last stand on the Templo Mayor (the great pyramid) and died falling from it. Pyramid-temples were present in many pre-Columbian cultures (including Aztec, Maya, Muisca, Chimu, Mound-builder etc. etc.)
Explanations?
Are there other explanations? Did the two disconnected societies just develop along different paths with pyramid-temples being present in Pre-Columbian American bronze age cultures but not in Old World pronze age? Did the presence or horses and mounted warfare in the Old World play a part (it seems difficult to efficiently make use of superior mobility against extensively fortified positions)?
Has this problem been considered by historians? Is there a debate about this? Is there more evidence or other crucial facts? Where can I read about this?
