The Observatory Tower is built on the smaller of the two mounds that abut the south curtain wall, which is dated a little later than the Lucy Tower. Excavation through the tower floor and into the mound itself showed that the rubble core of the mound and original tower foundations had been constructed in a single sequence in 12th-or possibly 13th-century masonry work.*

So when was the second tower built? A possible clue lies in the charter that King Stephen issued Ranulf, earl of Chester, which gave him leave to ’strengthen one of his towers in the castle of Lincoln’, apparently distinguishing this from the Lucy Tower.*

The quirky turret was added in the early 19th century, possibly as part of the then prisoner governor, John Merryweather’s rebuilding work here and on the Cobb Hall. Merryweather was a keen amateur astronomer.*

He had a handsome mounted telescope, and frequently spent whole nights in star-gazing, a very proper employment, I though, for the governor of a prison. One or two desperate attempts at escape had been promptly foiled by his vigilance…

Samuel Bamford, Passages in the Life of a Radical, 1840-44.*

*Lincoln Castle guide book – Scala Arts & Heritage Publisher Ltd 2015