The 1st Duke of Monmouth landed with his ship at Monmouth Beach, Lyme Regis, during his attempt to take the crown from King James II. He started a rebellion which failed. Many people were hanged right there on the beach and he himself was beheaded for treason. Some say the execution was pretty gruesome as it took a few blows to sever the head from his body. This is a story the guides at the Tower of London love telling.
The Duke's plan to take the throne from his uncle, was an idiotic idea. There at the beach the Duke met his doom which was the result of his own idiocy.
to enter upon something means …
...to begin or set out on something
We have entered upon a challenging period in our lives.
...to enter a place when something is happening
We entered upon a heated dispute when we arrived at our friend's house.
upon is a more formal term for on