In 2007-2008 a new excavation project was undertaken in order to resolve the chronological sequence of the Middle Wall at Co Loa, adjacent to the North Gate. This was the first full-scale, systematic investigation of any of the site’s fortification features. The objective was to begin recovering data and evidence that would provide clues about the site’s chronology, and to help determine the conditions under which political consolidation occurred. This was also the first Vietnamese and American archaeological collaboration ever at Co Loa, and the work is ongoing.
The team of archaeologists chose to excavate a portion of a rampart wall and its outer ditch. A 5 meter wide trench was cut through the entire width of the earthen rampart and into the defensive ditch outside the enclosure to the north. The stratigraphy in this trench showed several layers of construction deposits, which could be grouped into three periods and five major phases of construction. Stratified throughout the rampart were various artifacts and charcoal remains that allowed researchers to build a chronology for construction sequences.
Unexpectedly, a smaller set of architectural features was discovered at the very bottom of the excavation trench. These smaller-scale features may have been defensive and appear to be unrelated to the larger rampart construction that was to follow. Dongson pottery was found in association with them, along with charcoal samples that suggest construction around the fourth century BC.
The earthen rampart was constructed over the top of this smaller set of features, seemingly by a larger-scale and politically distinct society. This phase of construction likely commenced sometime during the third century BC, with the bulk of the rampart being built within a range of two to three centuries. The stratigraphy also shows that forms of stamped earth were used in the construction, and that the uppermost layers may have been refurbishment or amplification phases that occurred during historic times.