Celtic Druid pyramid, 2,700 BC, Britain.

Silbury Hill, a step pyramid constructed of chalk blocks.

Interior construction of 2,700 BC pyramid in UK.

Common misconception is that it is an earth mound, but it has a complex lattice construction.

Britain's first pyramid. Very likely of the Celtic Druid religion, being that the Celtic language arrived in the Islands at around 3870 BC, according to Toth. And there is no other religion, on the historical record, than Druidism, there.

Silbury hill pyramid. It is quite large.

"series of radial walls similar to a spider's web which divide the drums into compartments each of which are filled with chalk the walls themselves having been built with chalk blocks."

How many "mounds" have complex construction of walls?

Massive scale. Here are people at the top of the pyramid.

"This reveals that work on Silbury Hill began around 2,450 BC, and continued for between 55 and 155 years. ‘If you take the midpoint, which is a little over a century, we are looking at about four or five generations of people" https://www.archaeology.co.uk/articles/the-many-faces-of-silbury-hill.htm

"Mound" is a misnomer.

Definition.
"a large pile of earth, stones, etc. like a small hill"
https://dictionary.cambridge.org/us/dictionary/english/mound

A construction with walls is not a pile.

"As the diagram shows, the uppermost part was built as a series of drums each getting progressively smaller in diameter with increasing height. Each drum is about 17 feet in height with the outer wall leaning in at an angle for added stability." http://www.avebury-web.co.uk/silbury_hill.html