In October 1483, Henry Stafford, Duke of Buckingham, who had supported Richard’s accession to the throne, turned again him. His motives are unclear, but he threw his support behind Henry Tudor who was his second cousin. He planned to join forces with Tudor but was thwarted when Tudor’s forces were prevented by storms from reaching Wales (from France). The same storms stopped Buckingham from crossing the River Severn to join up with other rebels in the south of England, and his forces deserted him. He tried to escape in disguise, but was betrayed. The following month he was convicted of treason and executed.
So what has this to do with Grantham? Richard was at Lincoln when he heard of Buckingham’s treachery, and he sent a letter to the Lord Chancellor, demanding delivery to him of the Great Seal at Grantham. The letter, written in part by a scribe, has a footnote in Richard’s own writing, in which he declared Buckingham to be “the most untrue creature living.”
Richard’s visit in 1483 was also the origin of the carved angel holding a crown, which is above the original archway, leading to a courtyard and stables.
P.S. As a footnote to this post (I'm adding this on Monday evening), it is a somewhat strange coincidence that I should have posted an article about Grantham today. I wrote it sometime last month, and posted it about 11am, an hour before the news broke that the former Prime Minister who was born in Grantham had in fact died earlier this morning.