[1][2] The tumulus was designated a National Historic Site of Japan in 1923, with the area under protection expanded in 2014 and again in 2015, together with the nearby Koshitsukagomon Kofun (越塚御門古墳墳).

Long thought to have been an enpun (円墳)-style circular tumulus, it was excavated from 1977-1978 and was found to be a hakkaku-fun (八角墳)-style octagonal tumulus built in three tiers using tamped earth with an original height of 4.5 meters.The base of the mound had been carved into an octagonal shape with opposite sides measuring about 22 meters.

The stone paving remains themselves are bent at an angle of about 135 degrees in the middle to form a regular octagon.

When viewed from above, they have a common center and are made up of three tiers of mounds and three rows of flat stone paving, which are similar octagons of different sizes.

The outside of the stone paving is covered with gravel, and including this, the entire area is estimated to be about 32 meters in size.

Researchers believe that these cut stones were piled up in pyramids to decorate the slope of the mound, and they estimate that the total number of pieces is about 7,200.

This type of coffin found in a very small number of late Kofun period tombs, and is thought to have been used for the funerals of nobles as it was considered the highest quality of the time.

While the Imperial Household Agency Archives and Mausolea Department acknowledges that there is a theory that Kengyokozuka Kofun is a likely candidate for the tomb of Empress Saimei, it has no intention of altering its designations.

In October 2010, when the site of the Kengoshizuka Kofun was being backfilled, fragments of a stone burial chamber, were discovered .