The remains of more than 240 people, including children, have been unearthed by archaeologists working on the remnants of a medieval priory found beneath a former department store, reports BBC.
The "hugely significant" discovery was made under the old Ocky White building in Haverfordwest, Pembrokeshire. Archaeologists believe the ruins are from St Saviour's Priory, founded by a Dominican order of monks in about 1256.
Ocky White was a popular store for more than a century before its riverside premises closed in 2013.
Site supervisor Andrew Shobbrook, from Dyfed Archaeological Trust, described the priory as a significant complex of buildings with dormitories, scriptoriums - rooms devoted to writing and manuscripts - stables and a hospital.
"It's quite a prestigious place to be buried. You have a range of people, from the wealthy to general townsfolk," he said. It is believed that the graveyard could have been used until the early 18th Century.
About half of the remains are those of children, which is said to be a reflection of their high mortality rate at the time.
All the bones will be analysed by a specialist before being reburied on consecrated ground nearby.
One theory is that the victims could date from an attack led by Owain Glyndŵr, who was the last native Welsh person to hold the title Prince of Wales.
It was a joint assault by Welsh and French forces, who had united to battle English occupation of Wales.
"We know that the town was besieged in 1405 by Owain Glyndŵr and they could be victims of that conflict," said Mr Shobbrook.
The remains and other finds, including tiles, are being stored at a nearby disused shop after being cleaned and dried.
"Never in my wildest dreams did I think I would involved in something so big," said archaeologist Gaby Lester.
"The site is showing itself to be massive part of the history of Haverfordwest and Pembrokeshire.
"It can be slightly overwhelming at times but it's also quite humbling to be part of that person's journey."
The site is being redeveloped to become a food emporium, bar and rooftop terrace.