The castle and the South Blockhouse continued in use within the Citadel during the 18th and early 19th centuries.
In 1746, the South Blockhouse was redesigned with new embrasures, but the fortifications were largely neglected.
During the Napoleonic Wars, the Citadel was extensively repaired; the South Blockhouse was extensively altered to allow it to hold naval ordnance stores and the castle became an armoury, each wing able to hold 20,000 stands of infantry weapons and 3,000 cavalry arms.
The North Blockhouse and the remnants of the curtain wall beyond the Citadel were in ruins by 1766; the blockhouse was let to private contractors, and then demolished altogether between 1801 and 1802.

By the 19th century, extensive docks had grown up around the Citadel and in 1802 the surrounding land was granted to the Hull Dock Company.
The Citadel remained in military use until 1848, by when developments in military technology had made the fortification obsolete.
In 1858 there were proposals to turn the site into a public park, but instead the Citadel, including Hull Castle and the South Blockhouse, was demolished in 1864 to make way for an expansion of the docks.

From 1969 onwards there have been a range of archaeological investigations around the area.
The foundations of the Citadel, which had been too substantial to dismantle in the 19th century, were uncovered during urban regeneration works in 1987, and archaeological digs have occurred on both the castle and the South Blockhouse sites.
The foundations of these two buildings, along with the southern end of the Citadel remains, are protected under UK law as an Ancient Monument.
During excavations in 1997, an iron portpiece was discovered on the site of the South Blockhouse.
The weapon, now known as "Henry's Gun", is one of only four such guns in the world to have survived from the period and is displayed at the Hull Museums.
It was either made by Henry VIII's gun-maker or acquired from the Low Countries.
By 1681 it would have been obsolete and was disposed of in 1681 during the construction of the Citadel.
The castle and the South Blockhouse continued in use within the Citadel during the 18th and early 19th centuries. In 1746, the South Blockhouse was redesigned with new embrasures, but the fortifications were largely neglected. During the Napoleonic Wars, the Citadel was extensively repaired; the South Blockhouse was extensively altered to allow it to hold naval ordnance stores and the castle became an armoury, each wing able to hold 20,000 stands of infantry weapons and 3,000 cavalry arms. The North Blockhouse and the remnants of the curtain wall beyond the Citadel were in ruins by 1766; the blockhouse was let to private contractors, and then demolished altogether between 1801 and 1802. 
By the 19th century, extensive docks had grown up around the Citadel and in 1802 the surrounding land was granted to the Hull Dock Company. The Citadel remained in military use until 1848, by when developments in military technology had made the fortification obsolete. In 1858 there were proposals to turn the site into a public park, but instead the Citadel, including Hull Castle and the South Blockhouse, was demolished in 1864 to make way for an expansion of the docks.
From 1969 onwards there have been a range of archaeological investigations around the area. The foundations of the Citadel, which had been too substantial to dismantle in the 19th century, were uncovered during urban regeneration works in 1987, and archaeological digs have occurred on both the castle and the South Blockhouse sites. The foundations of these two buildings, along with the southern end of the Citadel remains, are protected under UK law as an Ancient Monument. During excavations in 1997, an iron portpiece was discovered on the site of the South Blockhouse. The weapon, now known as "Henry's Gun", is one of only four such guns in the world to have survived from the period and is displayed at the Hull Museums. It was either made by Henry VIII's gun-maker or acquired from the Low Countries. By 1681 it would have been obsolete and was disposed of in 1681 during the construction of the Citadel.
Hull Castle