These small coastal forts were essentially protected platforms on which to mount canon. In plan form, they were either quadrilateral, circular or semi-circular, while Worsley's Tower was exceptional in being octagonal. The square or rectangular platforms tended to be of earth construction with timber or stone revetments, while the circular ones are usually built of stone or brick.

Some idea of how these platforms were constructed is evident from fortification and military manuals of the time. The guns required a sturdy base to support them or else they would dig themslves into the ruts caused by the recoil. Consequently, platforms of stone or timber were recommended - more often, timber was used, as stone was liable to shatter. However, both Gurnard and Sandown Forts were both furnished with stone platforms.The stone gun platforms are clearly seen here in this picture of Sandown fort in the early eighteenth century.

A discussion about the relative merits of stone or timber platforms is clearly set out in a mid sixteenth century gunner's manual called 'The Gunner's Glasse':
Gunner The Platformes are to be well considered in a Castle or Fort, without the which, a man cannot make a good or perfect shot, and a man had as good as no peeces, as to do no good with them when we have them; for seeing, that in the reverse of the Peece, which is just at the instant that the powder taketh fire, then by meanes of the force of the powder, the weight of the shot, and the straightnesse of the wads, the peece is removed before the shot is out of the peece, by which meanes the Gunner is often deceived and makes a bad shot because the peeces do not equally reverse, and therefore Platformes are of great use.
Scholar Before you proceed any further, I pray you resolve me of this question, whether Timber or stone be fittest to make the Plat-formes, for I have heard much dispute amongst Artsmen about the same?
Gunner I have served with both Timber and Stone Plat-forms and do certainly know, that Stone Plat-formes are more serviceable and cheaper then Timber, and the Stone Plat-formes are farre more durable, and will out-last 10 Timber Plat-forms with very little repairing, I speak of Plat-formes in Forts or Holds.
Scholar What reasons can you aledge to prove whether Stone or the Timber Plat-formes are best?
Gunner My reasons are these, first I have known Timber Platforms to warpe, and become uneven by meanes of the heat of the Sun and the wet weather, and in wet and rainy weather to be so slippery that a man cannot stand upon the same; Also they are more chargeable then stone Plat-formes, and yet will not indure so long, for every 15 yeeres they do decay and must be new made, ...

[The Gunners Glasse by William Eldred. London, 1647.]
Another method is also used in forts, much more defective than this, which is, to build the parapets of brick or stone only, without any earth behind it; for as soon as the ships come once within reach of the fort or batteries, they may easily destroy this parapet, and the pieces of stones or bricks will be more dangerous to the defenders than the enemies shot itself. And it has been found by experience, that nothing disheartens troops placed behind a wall so much as the pieces of stone flying about their ears, and therefore such a practice should be avoided as much as possible; ...
[A treatise containing the elementary part of fortification, regular and irregular, John Muller. London, 1782.]

Concerning the construction of platforms, it is clear that by the 17th century there was broad agreement amongst fortification manuals:

Plan of a platform, showing planking arrangements.
Section through battery in Fig. 4 & 5.
[An universal military dictionary, Capt. George Smith. London, 1779.]

Below: Plan of a battery showing parapet, embrasures and gun platforms.
[Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers. Ed. D. Diderot. Paris, 1751.]

Above: Plan and section through a gun battery.
Concerning the Plat-forms in Castles and Forts, you must make them at least 17 foot in breadth, whether they be Timber or Stone, and in that space the Wheeles and the tayle of the Carriage will well stand upon the Plat-forme, and let the Plat-forme be higher behind than it is before, by 4 Inches, that is, let it better 4 inchesin seventeen foot, which is done for the ease of the Gunners in bringing their peeces up again to their places; as also to checke the reverse of the Peeces, but let it not be above 4 inches in seventeen foot; for I have seen some Plat-formes to be so high behind, that that the peeces have runne forward of themselves, and the Gunners must then put them back before he can load againe.

Now for the length of the Plat-form, let it be according to the number of the Peeces that you mean to place upon the same, keeping the distance of 6 foot, or at least 5 foot betweene Wheele and Wheele; as if you will place 10 Peeces upon a Plat-forme, you must make your Plat-forme 110 foot long, and let it be no higher at one end, then at another; and this course is sufficient to be observed in all Forts, both for Stone and Timber Plat-formes; where the Ordnance shall remaine constantly in one place.
[The Gunners Glasse by William Eldred. London, 1647.]

They [forts ] are often made of various figures, regular or irregular, sometimes in the form of a semi-circle, especially when they are situated near a river, sea, or at the entrance of a harbour; by which figure, they are able to fire at the ships on all sides of it; ...
[A treatise containing the elementary part of fortification, regular and irregular, by John Murray. p. 207. London, 1782]
How all platfourmes for great Ordinance ought to be covered with woodden planks: and how it is better to plant great Ordinance uppon plaine and levell platfourmes, than uppon slope platfourmes.
Cover all platformes for great Ordinance with thicke, & smooth, woodden planks joined close together : for if great peeces of artillerie be in time of service planted uppon a floore of earth, the wheeles of their cariages may with recoiles so sinke into the ground, that gunners shall not be able to manage well the same peeces. And though some platfourmes for great Ordinance are high behind the carriages of peeces, and loe where the wheeles of the same carriages shal ordinarily stand, to this end that the peeces lying in their carriages uppon the same slope platfourmes may after they have recoyled bee speedily brought againe into their ordinary places, yet (as many expert men in their gunners art do thinke) it is better to plant great Ordinance uppon plaine and levell platfourmes, than uppon slope platfourmes, because the great peeces of artillery which shall bee discharged from such slope platfourmes will oftentimes shoot short of their markes.

[Three bookes of colloquies concerning the arte of shooting in great and small peeces of artillerie by Niccolò Tartaglia. London, 1588.]
Works that are usually made within the Forts, as Platforms, Cavaleers or Mounts, and Cazemats in the Flanks; ...
1. Of Platforms. These are the Plantations where guns are to be placed, and are commonly made of Plank and Sleepers to lay them on; there needs not for one gun to be but one Platform, unless Timber and Plank be reasonably to be had, then you may make whole Platforms throughout, for so many Guns as you intend to plant together : As for a single Platform, next the Port-hole it need not be above 8 Foot wide, and the further end 14, the length ought to be 18 Foot at the least; the Parapet must be opened for the Port-hole, and need not towards the Gun be above 4 Foot wide; and towards the Campaign [countryside] or Enemy 8 wide, this called an Embrasure ; they are generally no more than 20, 18, or 16 Foot asunder from midst of Port to midst of the Port : But if the Guns be to Play clear over, without any Ports, the Brestwork must be abated to 2 1/2 or 3 Foot high, as the Wheel of the Carriage will be high, and instead of Earth you must have Cannon Baskets and Earth ready by, to fill, set, and remove, as you shall have occasion.

[Modern fortification, or, Elements of military architecture by Jonas Moore. London, 1673.]
The PLATFORM is a floor made to facilitate the rolling of the carriage wheels, and to prevent them from sinking into the ground by the weight of the cannon, especially in wet weather.
Platforms are generally laid sloping toward the parapet 9 or 10 inches; this carries off the rain, prevents the gun from recoiling so much when fired as it would do if laid level; and when loaded, it is more easily brought to the embrasure. In temporary batteries the platforms are made of planks laid across some ground timbers or sleepers; there is usually a platform made to each gun; it is about 18 feet long, 8 feet broad next the parapet, and about 14 feet broad at the tail, the intermediate spaces between the platforms serving for the shot and other necessaries. When a platform is to be laid on marshy ground; first lay a floor or two of fascinex; cover these with hurdles of 12 or 15 feet long, and 6 or 7 broad; on these lay a floor of 3 or 4 inches of earth, in which the sleepers are to be laid, and over them the planks. When a battery is built of stone or brick, the platform is generally a flat-stone pavement ranging the whole length of the battery. This, on account of its resisting the injuries of the weather for a long time, is to be preferred to planks; but in case of a bombardment such a platform is to be avoided, because the shells will not only break the pavement, but also, by driving about the broken stones, do the troops much mischief.

[A treatise of marine fortification. by J. Robertson. London, 1754.]