This was considered favourable and circular staging , resting on piles screwed as far as possible into the clay, was erected for the purpose of tipping material to form the foundations. The piles formed concentric circles of 112 and 56 feet in diameter, and a radial traveller worked between them. The rubble foundations consisted of the hardest type of Portland stone, the largest pieces not exceeding 2 tons and the remainder being small chippings, rubble and grit. Every truckload contained this mixture, which was intended to be worked together by the action of the waves into a compact mass.

In 1861 a modified design of only two tiers, carrying 35 guns, was proposed and the next year drawings were sent to Portland showing the arrangements for the basement storey. There were to be 22 radial walls projecting inwards from an 11 foot thick external wall of Portland stone faced with granite. As the intended Fort was to be a masonry work with a large interior parade ground, the materials of the outer ring were laid with more care than those in the centre. By the end of 1862 the work had been brought to a level 20 feet above low water.

The first difficulties arose in October 1864. A portion between some piers had broken away from the remainder and at one pier a subsistence of 2'10" had occurred. It was thought that the cause was likely to be a thin hard stratum or a yielding bottom under the part that had given away, aggravated by stormy weather. Additional material was placed at the base of the slope.

Breakwater Fort - further difficulties

Building of the masonry fort would probably have begun at the end of 1866 but for the uncertainty that still existed as to the stability of the foundations and that no plans had yet been approved for the superstructure - so construction was put off year by year.

Ten days of severe storms in January 1867 produced further cracks in the masonry, and further gales in March caused a breach in the outer wall. The Royal Engineer in charge considered that the movement was almost entirely due to the combined action of the subsidence and consolidation of the material composing the foundations. At last in 1867 a decision on the superstructure was made. The Fort was now to be an iron plated work of much smaller diameter, which would consequently not rest on the already built masonry foundations. New foundations were to be prepared by removing the loose hearting from the centre and replacing it with a solid cylinder of concrete 10 feet thick and 122 feet in diameter. The removal of the rubble began in 1868 and by the end of 1869 the task was completed. Meanwhile the settlement continued.

Amended 1869 plans envisaged an armoured fort in two tiers of casemates mounting 29 guns, with alternative designs for a single tier or an armament of four turrets. Turrets had been seriously considered for the Spithead Sea Forts and even for the Nothe. In view of the concern over the foundations it was recommended giving up the idea of a two tier fort.

Breakwater Fort - the armour arrives and difficulties continues

Another revised plan in 1870 called for a lower tier of 14 guns supporting an upper tier mounting 7 guns, and the superstructure was begun on this basis. The upper tier was afterwards abandoned. The iron skeleton, made by Jeavons & Co. of Millwall, was assembled in place in 1873. The armour plating, of three layers each 61/2 inches thick, was made by Messrs. Brown of Sheffield. It was taken by rail to Gosport, and then by water to Portland - to the Fort jetty, where it was unloaded by an overhanging timber gantry with a traveller. The traveller dropped each piece into a purpose-built truck resting on four lines of rail which was drawn up an incline to the outer circle of the Fort, where it was again picked up by another traveller working on a circular gantry and placed in position. The spaces between the armour plates were filled in with fine Portland cement concrete, except at the gun ports where they were closed by teak.

Subsidence continued to take place as these massive weights were piled on. The difference of level between the highest and lowest points of the outer wall was about 11 1/4 inches. In addition to this fresh subsidence began under the massive superstructure, causing the bed of concrete to assume a saucer shape and leading to various cracks and breaks in the masonry. However, by 1878 this movement appeared to have very nearly ceased. The only extra weights now to be added were the armament and ammunition, rather less than 1000 tons and, as this was to be evenly distributed, it was not thought likely to add materially to the subsidence. Concern was felt, however, about the result of very severe storms, one in November 1876 having had a marked effect, contributing to further yielding of the sea bed under the weight of the work. Another concern was the washing away of the foreshore on the exposed side. In 1871 this had been brought up to above low water of spring tides, but by 1878 it was in parts 20' below that level. The material added in 1870 which had disappeared was in blocks of no less than 3 1/2 tons and some approaching 6 tons.

Breakwater Fort completed at last

By January 1878 the Fort was on the point of completion, and the armament of fourteen 38 ton guns was expected, but they had not arrived by 1882. The masonry of the Fort, like that of the Verne and the Nothe, had been of Portland stone, but a great deal of work had been done in concrete. The stone used for the concrete was Portland from various beds, broken down to the size of road metal, while the sand mostly came from Moreton, a particularly sharp and clean sand. All masonry and brickwork was set in Portland cement, not common lime mortar. Great care had been taken in manipulation of the concrete and the results by 1878 had proved very satisfactory, seas having broken with great force on the concrete apron with no damage whatever being caused.

The Fort was almost instantaneously obsolescent. Advances in both guns and armour plate meant that it was defective both in attack and defence. Although spaces had been provided in the structure of the Sea Forts to add more armour, the matter does not appear to have been considered, and the heavy breech loading guns which the French Navy had adopted quite outranged the Fort's RMLs. The relative uselessness of the Fort was recognised, and by 1895 six of the RML had been removed, incredibly from the seaward side.

Records from exercises of 1895 show the Fort with seven 12.5 inch RML, worked by Royal Artillery and two 6 pdr QF worked by Sligo Artillery Militia. The RML had no shrapnel shells, and so to be employed against torpedo boats at night they had to be kept loaded with case shot and laid with an elevation of 800 yards to be able to fire at a moment's notice. By 1900 the Breakwater Fort was armed with four 12.5 inch RML, six 12 pdr QF and three machine guns. Temporary huts were erected on the masonry apron, and an engine and dynamo for seachlight positions built on the Breakwater. Buildings for housing and operating boom defence equipment were also provided. The success of a black and white chequer scheme in camouflaging Spitbank Fort led to all the sea forts, including Breakwater Fort, being similarly treated as a consequence, and the Portland example is still popularly referred to as Chequer Fort. A War Office Committee visited the Fort in 1905 and recommended mounting two 6 inch guns on top of the Fort and two more in the casemates - however it was decided to mount the two latter on the heads of the new arms of the Breakwater. The QF were to go. In 1907 the removal of the 12.5 inch guns began, and measures were taken to emplace two 6 inch BL on the roof. By 1909 two 6 inch had been mounted there.