During archaeological excavations at 18 Hoang Dieu Street, archaeologists discovered overlaying cultural layers representing the uninterrupted continuation of a long historical process. The bottom cultural layer is the remains of Early Thang Long, also known as An Nam domination or Dai La. At this cultural layer, archaeologists have found the remains of wooden column system, architectural foundation, sewer, drainage and water wells.
Quite a lot of artifacts of Dai La time are also found at this cultural layer in all 4 areas. They are rectangular bricks, square tiles, yin-yang tiles, tubular tiles, heads of tubular tiles decorated with holy beasts, buffon faces, architectural roofs decorated with big beast heads and ceramics. The characteristics of these types of archaeological remains are that they were made of dark gray clay; rare types were made of red clay.
Dai La wells were dug according to the order of Northen domination mandarins when they occupied in Dai La Citadel. Dai La wells found in 18 Hoang Dieu archaeological area had the depth of 5.9 m. The wells were built with good quality gray bricks; each vertical brick line was alternated with four horizontal brick lines. Dai La ancient wells were built by arranging bricks close to each other without using any cement material. This brick layout is often found in many ancient works. Most of the wells built during the Northen domination in Dai La were utilized by King Thai To Ly Cong Uan to save the national treasure and make convenient for moving the capital in a rush.
The bricks found in Dai La cultural layer are engraved with the words “Giang Tay Military”, so they are called Giang Tay Military bricks. Historians think that Giang Tay Military bricks were produced by Giang Tay military under Tang Dynasty during the time they occupied Thang Long. These bricks were very durable, so later, Vietnamese dynasties often took advantage of these bricks and combined with bricks manipulated by Thang Long craftsmen to build palaces.
This is the first cultural layer among the overlaying cultural layers at 18 Hoang Dieu archaeological area to be found. The layers above this are Ly – Tran cultural layer and other cultural layers.