Northumberland was armed with a mix of , , and rifled muzzle-loading guns. All 4 nine-inch and 18 eight-inch were mounted on the main deck while 4 eight-inch guns were fitted on the upper deck as chase guns. Both seven-inch guns were mounted in the stern on the main deck, also as chase guns. The nine-inch gun was credited with the ability to penetrate of wrought iron armour at the muzzle. The eight-inch gun could penetrate of armour and the seven-inch gun could penetrate . Northumberland was partially rearmed in 1875 with an armament of 7 nine-inch guns, 4 on the main deck, 2 forward chase guns and 1 rear chase gun. Two eight-inch guns replaced the seven-inchers on the main deck at the stern; the other 18 eight-inch guns remained where they were. In 1886 two breech-loading guns replaced two eight-inch guns. Six quick-firing (QF) 4.7-inch (120-mm) guns, 10 QF 3-pounder Hotchkiss guns, and six machine guns were later added. Armour Unlike her half-sisters, the entire side of Northumberlands hull was not covered with wrought iron armour. To compensate for the additional weight of her armament, only her battery was protected above the main deck. The ship was fitted with a complete waterline armour belt that tapered from at the ends to amidships. The armour extended below the waterline. The sides of the battery were long and it was protected by 5.5-inch armour on all sides, including transverse bulkheads fore and aft of the guns. The two forward chase guns on the upper deck were also protected by armoured bulkheads, but the stern chase guns on that same deck were entirely unprotected. The armour was backed by of teak. The ship also had a conning tower protected by 4.5-inch armour plates. Construction and service Northumberland, named after the ceremonial county, was ordered on 2 September 1861 from the Millwall Ironworks. She was laid down on 10 October 1861 at its shipyard in Millwall, London. She was altered while on the building slip after Sir Edward Reed succeeded Isaac Watts as Chief Constructor. Unlike her half-sisters, the ship spent five years on the stocks before she was ready to be launched, partially due to frequent changes in design, although Northumberland was much closer to completion. The additional weight caused her stick for an hour on the slipway before she slid halfway down with her stern only supported by air, threatening