In 1588 the fortress was garrisoned in expectation of a landing by the Spanish Armada. It figured, too, in the Civil Wars of the time of Charles I. _Holy Island._--Of the two castles on Holy Island, one, known as the Fort of Beblowe, was erected in or soon after the year 1539, and doubtless belongs to the great series of coast defences set up by Henry VIII. The other castle belonged to a subsequent period, and is believed to have been built in 1675. It is a remarkable fact, that of all the blockhouses built on the coast, or even in the estuaries of rivers, by Henry VIII, built, as we know from documentary evidence, at enormous cost, there is absolutely no record of any of them having been of real value in destroying the enemies' shipping. From some not a single shot was ever fired, except, perhaps, during the Civil War, when King and Parliament were at variance, and also upon the occasions of public rejoicings, such as royal birthdays, proclamations of peace, etc. It says much for the intimate knowledge of the distribution of our defences that the Dutch, when they invaded our shores in 1667, steered clear of these castles, and made straight for the Medway, rather than for Portsmouth or Dover, or the east coast of Kent, where there were castles of the Henry VIII period, and later, guarding the shores. One point in the construction of these blockhouses which must arrest the notice of every one who pays any attention to the subject, is the excellent illustrations they afford as to modification of military architecture due to the introduction of gunpowder. This explosive substance which revolutionized military tactics as soon as the art of using it and of making suitable fire-arms had reached perfection, was probably invented or discovered in the thirteenth century. For many years, however, its possibilities were imperfectly understood, and its employment was more dangerous to those who used it than to those against whom it was used. The castle-building towards the end of the fourteenth century--say the reign of Richard II--was distinctly influenced by the new force employed in attack and sieges. Curves become the fashion instead of flat walls, and by the sixteenth century, when Henry VIII erected this great series of blockhouses, we find that every means was taken to avoid presenting much surface to the action of cannon-shot. The walls were all curved to ensure the shot glancing off, and the whole structure was sunk in a moat, and built in very strong masonry, and with no more height than was necessary.