Kofun (ancient mounds) refers to the huge burial mounds built in Yamato after the middle of the third century and are distinguished from funkyu-bo (knoll mounds) of the Yayoi period. Many kinds of mounds were constructed but the characteristic hallmark landscape of the Kofun period is that of the “keyhole tombs” known as 前方後円墳 zenpo kofun in Japanese.
The large tumuli of the Kofun period did not suddenly appear, but likely had their model in the large hillside mounds of the earlier Yayoi period.
Pre-Yamato period mounds
From the middle Yayoi to the early Yamato period, rather large mounds (funkyubo) were being constructed on hills or knolls from a zone extending from Chugoku to Shikoku in the west to the Kanto plain in the northeast. These were either square in shape and surrounded by moats and ditches, similar to those found in China and north Asia.
Before keyhole shaped mounds became the standard, a wide variety of mounds of differing shapes and sizes could be found all over Japan. These can be classed into the main types:
- empun: round tomb mound;
- zempo koen fun: front-square and rear-round tomb mound;
- zempo koho fun: front-square and rear-square tomb mound;
- hofun: square tomb mound
By the late Yayoi period however, some mounds became larger (between 40 to 80 meters long) and a few of the late 3rd century mounds are known to have evolved the keyhole shape (e.g. Hashihaka and Hokenoyama and Kurozuka mounds) similar to those typical of the later Yamato mounds.
Pre-Yamato mounds with haniwa clay tubes and stone cist burials have also been found at the Makimuku site in Sakurai (Nara Prefecture) and as far east as Ichihara in Chiba Prefecture.
The early mounds were probably built at a stage for leaders who had brought several agricultural communities under their control but had not yet accumulated power and authority equal to that of a Yamato king.
Round mounds present from the beginning of the period, served as burials for lower-ranking aristocrats. By early 6th century families of clan leaders were buried in round mounds in what were clan cemeteries clustered on hillsides.
Keyhole shaped mounds become standardized
The famous large keyhole mounds did not appear until late Yayoi or the beginning of the Yamato period. The earliest among them emerged in the southwestern corner of the Yamato Basin near Mt Katsuragi and in the northeast at the foot of Mt Miwa. Eventually they spread out along the Yamato River, and then to faraway corners of Japan such as Echizen, Izumo. Keyhole mounds in Kyushu (the largest of which are the Mesahozuka and Osahozuka mounds in the Saitobaru mound cluster) have been found as well, dating from the middle half of the 3rd century. The keyhole mounds were built in quick succession, often in lines or clusters, eventually becoming the standard sort of mound for the highest ranking rulers and kings.
Why were the mounds keyhole shaped? It is a matter of conjecture … one theory is that the shape looks like a horse’s hoof, more likely, the design evolved its “keyhole-shape” form after experimentation with various shapes.
The keyhole mounds also grew in size. This indicated that the workforce and number of labourers available had grown as well … either because of a burgeoning local population or because of a large influx of immigrant workers, or perhaps due to a combination of both. And the presence of increasingly larger tumuli indicated the presence of increasingly powerful rulers in the area.
Haniwa cylinders from the Sakitama Burial Mound, Saitama Prefecture
Clay cylinders called haniwa were often half buried in the tomb surface encircling most of the tombs. While early haniwa were simple earthen cylinders, many of the later haniwa were sculpted into extremely artistic and aesthetically pleasing shapes and objects that tell us a lot about the kofun people and daily life.
Characteristic mounds of the Yamato kings – at the foot of Mt Miwa
In the Shiki area at the foot of Mt Miwa (in the southwestern corner of the Nara plainI, six mounds were built from AD 250 and 350: the Hashihaka mound (Sakurai); the Nishitonozuka (Tenri city); the Tobi Chausu-yama of Sakurai; the Mesuri-yama (Sakurai) and the Suijin tomb (Ando-yama, Tenri city) and the Keiko Tomb (Shibutani Muko-yama).
The tombs…
- are exceptionally large (twice as large as any tomb found in Korea);
- were all built in quick succession, one after the other, in the Shiki area;
- contained impressive coffins made of split bamboo and pine and surrounded by lavish grave goods: large numbers of mirrors, weapons, tools and ornaments.
As such, these Mt Miwa tumuli have been identified as the tombs of powerful priest-kings kings, rulers of a new and expanding Yamato kingdom.
Burial facilities
Early on in the Kofun period, the deceased person was placed in a wooden coffin buried directly in the tomb summit, or in a pit lined with stone slabs, and then covered over with ceiling rocks. Later, stone coffins were used.
In the late Kofun period, stone chambers with horizontal entrance passages were constructed. This way, they could re-enter the chamber and add more burials of family members later.
At the end of the Kofun period, the culture of building massive kofun tumuli began to die out due to resources being diverted toward temple-building instead. Square mounds then became the preferred shape of mounds for the highest ranking aristocrats all over Japan.
Heritage of Japan 
Were Haniwa tomb guardians every cast in iron?
None to our knowledge. There are no reports of iron tomb guardians though if there had been they would likely have been melted down, reused, or grave-robbed. Iron was precious and used for weapons and tools.