During the course of the 16th century, gunpowder-dependent artillery increasingly defined the thinking and technology of fortifications, resulting in a visible change in their physical shape and structure. By the latter part of the century, a clear set of consistent rules was beginning to emerge, but with some minor national variations. By the 17th century, fortification building had become an exact and internationally standard science, based on printed manuals and professional fortification designers, such that the construction of a fort was more a question of geometry and off-the-peg standard design than art based on individual initiative. During the first half of the 16th century, various different fortification solutions were developed in answer to the threat of gunpowder artillery: the blunt, rounded artillery tower; earthwork platforms; angle bastions; and extended belts of casemates and sconces around the old town walls.

The first point to note is that these minor forts were very small in size. Important centres, such as ports, significant waterways and major towns were defended by large, permanent forts that often still exist today. However, all round the coasts of Britain,small forts and batteries were constructed to defend minor places of significant strategic importance during a time of crisis. Often these were not intended to be permanent, but to answer a temporary need. Consequently, they were allowed to fall into disrepair, only to be refortified in a new time of danger from invasion. From the mid 17th century this possibility became more and more remote with the development of a strong Royal Navy, such that, during the 18th century, British defensive policy shifted to reliance on a navy to defend Britain's shores rather than a defensive string of fortifications. Consequently, the Island's minor forts, especially those made of timber and earth,were allowed to decay to nothing, while some fell prey to erosion. By the 18th century, the large, technical, modern armies required a port at which to land its huge amount of equipment efficiently. Small creeks and beaches sufficed the small, simple raiding units and armies of medieval times, and this was the case right up until the 17th century. Technologically deficient, but with high mobility, these small armies were able to land in very minor places: bays,sandy beaches, creeks etc.

Coastal fortifications are therefore essentially level gun platforms, on which cannon are deployed and fulfill two functions: one form concentrates on controlling a body of sea, such as a road, harbour, channel or strait, while the other focusses on denying a landing place on a shoreline to an enemy.

Calshot Castle at the mouth of Southampton water. An example of a fortification built to deny access by enemy ships to a channel and harbour. To highlight this point, the artist has drawn a ship being sunk by the cannons of the fort.
[BL: Cotton Augustus I.ii, f.106]

Sandown Fort on the Isle of Wight. An example of a fort sited to deny the enemy access to a favourable landing site.
[BL: K top Vol 15, 36]
The function of these small forts on the Island was to act as platforms for guns of various calibres and thereby to protect landing places in a defensive capacity or to attack threatening enemy ships in an offensive capacity. These types of coastal fortification were usually built close to the waterline so that they had a clear and full field of fire at sea level. Examples of this is evident in the various fortifications at the foot of cliffs: Dover, Pendennis, Dartmouth, and the gun batteries built at the foot of the Needles cliffs. Not only did this eliminate a dead area that might exist if the fort was built in a higher position (guns had a limited amount of depression ), such as King Charles Castle, Tresco, but also it allowed guns to be fired more efficiently at the hull as both point blank and ricochet fire could be used.

It's not until the 18th century that fortification and artillery manuals actually specifically talk about coastal defence but the principles were the same as applied to 16th and 17th century coastal fortification.

For as the batteries on shore are seldom raised much above the water's level, in order to be more certain of their mark; the troops posted at the guns are liable to be commanded by the marines quartered in the round-tops, and other elevated parts of the ships. However, this may be prevented, either by driving several rows of piles ( which are long pieces of timber, or the bodies of trees, pointed, and shod with iron at one end when necessary) in the water before the fort ; ...
Thus the soil should be the firmest and most capable to bear the cannon and building to be erected. The ground of a height sufficient to be out of the reach of tides and floods, and not much higher than is barely necessary to avoid those inconveniences; for the canon commands best when near the water's level. The place should be difficult for an enemy to land in its neighbourhood; and if landed, where they may be most incommoded in their march to the fort ...

[A treatise of marine fortification. by J. Robertson. London, 1754.]
Of Sea Port Towns.
Maritime towns are fortified, on the land side, like other places; but, on the sea side, ramparts are made, with cavaliers upon them, to keep the enemy at as great a distance as possible: below these ramparts batteries are erected just above the water, that the cannon may graze the surface, and so more effectually destroy the ships of the enemy in their approach.

[Elements of Fortification, Lewis Lochee. London, 1780.]

Of Forts and Batteries for the Sea Shore.
224. The Situation of Forts and Batteries for the Sea Shore, for commanding Harbours, or Places of landing, and the Channels leading into them, must be determined chiefly by the Situation and Course of these Channels with respect to the Shore, or small adjacent Islands, Rocks, or Keys, or Points of Land projecting into the Sea, that they afford convenient Ground for raising Batteries on that will most effectually command the said Channels, and present, at the same Time, a considerable Front to the Harbour.
225. Our Batteries on the Sea Coast are generally made with very low Epaulments, and sometimes wholly unscreened, and open towards the Field; ...
228. .. the Fire of the collateral Batteries cannot fail of having the best Effect, their Platforms being on a Level with the Surface of the Sea at high Water.
229. Again, if we suppose a small Island Island to have one Harbour, and five or six little Bays where Troops may be landed in Boats; it is not to be imagined, that the Government or Prince whose Property it is will be at the Expence of raising a defencible Battery or Fort at each of those Places of landing, unless the Island is of very great Importance indeed: It will be sufficient to raise one Fort for commanding the Harbour, with collateral low Batteries, if found necessary; and to throw up before each of these Bays, in time of War, a Battery of Earth, properly reveted, and bound with Fascines, which may be closed on the Side of the Land by a small Retrenchment; and it may be reinforced and sustained, and the Bay more effectually commanded by collateral Retrenchments that shall flank the Battery, and embrace the small Bay, or that Part of it where the Enemy can most conveniently land; the Ditches of these Retrenchments, and before the Battery, being well pallisadoed.
230. Where there is deep Water very near the Shore, whereon a Fort is to be raised for commanding a small Harbour, or any Place of landing, it will be of the utmost Consequence, tho' contrary to the common Practice, to raise the Fort at some Distance from the verge of the Water, if the Ground will admit of it, so as that a Ship of the Line shall not be able to approach within three hundred Yards of the Fort; for the Uncertainty of firing from Ships of War increases in a very great Proportion, as the Distance is enlarged, by reason of their incessant Motion, while Forts and Batteries erected on the firm Land are void of that Inconvenience: ... But when a Fort is intended to command a Channel leading into a great Harbour, it may be erected nearer the Shore, especially if the Channel is broad, that Ships which attempt to pass it may run a greater Hazard.
231. Nor will this Wall ever be in Danger of being destroyed, or breached by Ships of War; which is well known to those who have been at the attaking of Forts from the Sea ... since Men of War very seldom bring to before a Fort nearer than 150 Yards; and every one knows the Uncertainty of firing at a small Mark, at that Distance, from the Side of a Ship that is continually in Motion from the incessant Agitation of the Sea, even in the most perfect Calm, and which is increased by the Shocks occasioned by the recoiling of the great Guns: ...
232. The Port-holes of the Sea Fronts of these Forts must be made as narrow, for the Reasons already mentioned, as is consistent with the Use of them : The Dimensions of such a Port may be as follows; the Thickness of the Wall being supposed eight Feet at the Bottom of the Port : The horizontal Breadth of its interior wide Extremity may be six Feet; its perpendicular Height three Feet; the Slope of the Bottom of the Port from the interior to the exterior Surface of the Wall one Foot; the Perpendicular Height of the exterior Mouth of the Port three Feet; its horizontal Breadth also three Feet; and the horizontal Breadth of the Port, at three Feet from its exterior Border, two Feet six Inches.
[The theory and construction of fortification. Illustrated with several new designs by Lieutenant Charles Bisset, ... London, 1751.]