Thanh Nha Ho (Citadel of Ho dynasty) was a typical and great imperial city in the Vietnamese history. It was also a place for a significant interaction between Vietnam, South East and Southeast Asia in the historical context between the late 14th century and early 15th century. Regarding architectural planning, Thanh Nha Ho represents not only traditional acquisition and overseas interaction but also innovative and original elements that create extraordinary values of this architecture. One of them was the construction of Hoang Thanh (Imperial Citadel) and Nam Giao open-air altar with huge rocks in a large scale that has never seen in Viet Nam and any other countries in the region. In terms of building techniques, the huge rocks were exploited and refined possibly in the rage of mountains near the site. However, the transformation and overlapping of tens - ton rocks onto each other with highly accurate rate are actually wonder. This reflects ability of scientific measurements, originality and skillful hands of designers and builders of Dai Viet era, an exceptional phenomenon in the history of Vietnamese architecture.