After discovering the illustrations from "Armamentaria" of the Dutch Marines, my interest in the attack on Landguard Fort, as first reported by Ralphus here, has been renewed. After a bit of searching, I found a few sources that those interested in this battle may find useful.
The first is from a site called "Fortified Places", which can be found here, and provided the illustration above of the Fort as it might have appeared in 1667, after Charles II improved the Fort's defences in 1666.
The article is well worth reading in its entirety as it covers the complete history of the Fort and its many "generations". However, the "meat" of the portion that concerns the 1667 attack is excerpted here:
In 1625 construction of a new fort on Landguard Point was started, in conjunction with a new battery at Harwich. It was a square earthwork fort with 4 bastions and a ditch. The fort was armed with 62 guns and could accommodate a garrison of several hundred men.
The fort was not maintained and erosion took its toll on the ramparts, but it was still garrisoned and the Parliamentarian's held it during the Civil War. In 1652 England entered into a series of naval wars with the Netherlands, placing Harwich on the front line. A dockyard grew up there in the 1650s, but Landguard Fort remained dilapidated.
During the Second Anglo-Dutch War England suffered a number of defeats and in early 1667 there were fears for the safety of the east coast. As a result the Dutch-born engineer Sir Bernard de Gomme* was sent to Harwich on an urgent mission to survey the defences and make improvements. Finding Landguard Fort in a much-decayed condition, he ordered work to be done to strengthen the fort as soon as possible.
The improvements ordered by de Gomme consisted of enlarging the bastions and restoring the slumped earthwork ramparts, as well as the construction of a "false bray", a second rampart running in front of the main rampart at a lower level, at the foot of the rampart all the way round the fort. The ditch and the false bray were revetted in brick.
*Sir Bernard de Gomme was responsible for the majority of fortification works undertaken by Charles II of England. Having served under Charles I in the Civil War, de Gomme was knighted and given a promotion when the monarchy was restored in 1660.
The next source, which I found quite interesting, was from GoogleBooks and is "The History of Landguard Fort In Suffolk" by Major John Henry Leslie, found here. This book has an excellent section on Landguard with more of a panoramic view of the fort (page 14) that matches the illustration above, except that it shows the main walls as being of either stone or perhaps sod blocks and describes the Fort as being of brick, which also matches the illustrations by Charles Stadden that Ralphus posted earlier.
Most sources agree that Landguard was built in 1628 of earthwork reinforced by wood, and that the brick works added by de Gomme in 1666 were only to reinforce the ditch and to create the lower "false" bray. While some of the Stadden illustrations could depict the hand-to-hand fighting at the lower bray, its unlikely that a gun on a naval carriage would have been positioned there. The Leslie book is an invaluable resource for anyone contemplating a model of Landguard Fort, or a game involving it, as it gives an exact breakdown by type of the 62 guns located within the Fort, along with a general treatise on guns of the period. As with all Google Books, this volume can be downloaded and saved as a pdf file, and does contain information on many other details of the Fort and its long history, including personnel and armament through the years.
For those that want to do some "serious" research in addition to the above, there is also an historical essay on the Battle of Landguard which can be downloaded from Coursework.info for £4.99 here, and an article from "The Journal on Post-Medieval Archaeology", Volume 42, Number 2, December 2008, pp.229-275, here for $39.00, that covers the recent archaeological excavations at the site down to the level of the 17th century works.
By the way, for anyone considering building a model of the Landguard Fort, there is a first-hand report in Leslie's book of the work-in-progress with references to the platte, or plans, with excellent dimensions. I worked out a quick rough calculation and in 15mm, not counting the proposed 60 foot wide ditch, it works out to about 60" square. In 6mm it would be slightly less than half that, about 29" square. This is based on actual height of typical figures, not advertised "scale". A large model to be sure, but not impossible for a club to consider for a convention demo. Alternately, you could model just the landward curtain wall and two bastions and do the majority of the actual attack by Dutch forces.