from Penny Drayton's treatise 'Toot Hills' -

Arguably the most auspicious toot hill is the one at Westminster, London. Westminster Abbey and the Houses of Parliament are the most-recent of a succession of palaces and churches which date back to the post-Roman period. Indeed, the first church here, dating to the seventh century, may have taken advantage of the copious remains of Roman buildings. The locality was known for many centuries as Thorney Island, being an area of relatively solid ground amid the marshes bordering the Thames. Additionally, there was an artifical mound, known as Tot Hill.

Tot Hill still stood in Queen Elizabth I's time, as Nordon, the topographer of Westminster, wrote 'Tootehill Street, lying in the west part of the city, takes the name of a hill near it which is called Toote Hill, in the great field near the street.' Toot Hill is indeed shown on a 1746 map by Rocques by a bend in Horseferry Road roughley where Regency Palace now stands. The name survived in Tothill Fields, the old tournament ground now part of of the playing field for Westminster School in Vincent Square, and Tothill Street, which aligns with the northern transcept of Westminster Abbey. Alfred Watkins discovered and described a pair of leys, one running down the middle of Tothill Street, although his claims that the two alignments crossed at Tot Hill does not match Rocques' map, although there is no certainty that his cartography was reliable.

Jeff Saward has recorded that there was a maze on Tot Hill, which was recorded as restored in 1672 and traditionally a site for various games of skill and agility, and a so-called Troy Game (played every Sunday in Lent by knights on horseback) may be first recorded in the sixth century.

Sir Thomas Mallory, in the fifteenth century, has Queen Guenevere inviting the Knights of the Round Table to ride out early one morning in May into the woods and fields beside Westminster. Such specualtion about earlier activities here was kept alive throughout the nineteenth and early twentieth century by persistent specualtion of Tot Hill being a Druidic site, although the origins of this fable have been lost in the proverbial mists.

The trench ontop showed a series of stones walls creating spokes that were flat across the top of the hill.
The spokes were filled with chalk blocks.
There are some good photos in whittle's book.
Good chance the top was meant to be flat eh?