From 2009 to 2012, under the research project of Hoa Chau citadel, 17 pits were test-excavated in the ancient Citadel. The results demonstrate that Hoa Chau Citadel consisted of just two ramparts, of which the external rampart is 4,700m in circumference. In terms of architecture, Hoa Chau citadel also has its own feature as compared to Champa citadels, as it has 2 - 3 defence enclosures at the comer. In the citadel area, there are settlement traces of Sa Huynh - culture inhabitants on raised mounds in the marshland. The high land adjacent to the northeast and southwest of the internal citadel was built in the Champa and Tran periods. The fact that its scale was not large and there were not many the cultural remains of Champa period indicates that Hoa Chau citadel can't have been the Imperial city, but functioned as military or transport base at this time. In the Tran - Ho period, the architectural traces larger in quantity and scale, together with the cultural remains having clear Viet characteristics from the fifteenth to eighteenth centuries.