Star fort

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Bourtange fortification, restored to 1750 situation, Groningen (province), Netherlands

A Star Fort is a fortification in the style that evolved during the "Age of Blackpowder" when cannon came to dominate the battlefield. Passive ring-shaped (enceinte) fortifications of the medieval era proved vulnerable to damage or destruction by cannonfire, when it could be directed from outside against a perpendicular masonry wall. Star fortifications were developed by Filarete in the mid-fifteenth century, employed by Michelangelo in the defensive earthworks of Florence, refined in the sixteenth century by Baldassare Peruzzi and Scamozzi and brought to final statements by Louis XIV's military engineer, Vauban. In the nineteenth century the development of the exploding shell changed the nature of defensive fortifications.

The star-shaped fortification had a formative influence on the patterning of the Renaissance ideal city: "The Renaissance was hypnotized by one city type which for a century and a half— from Filarete to Scamozzi— was impressed upon all utopian schemes: this is the star-shaped city"^[1]

1 Origins
2 Notes
3 See also
4 External links

[edit] Origins

When the newly effective maneuverable siege cannon came into military strategy in the fifteenth century, the response from military engineers was to arrange for the walls to be embedded into ditches fronted by earth slopes so that they could not be attacked by direct fire, which is by far the most destructive, and to have the walls topped by earth banks that absorbed and largely dissipated the energy of plunging fire. Where conditions allow, as in Fort Manoel in Malta, the "ditches" are cut into the native rock, and the "wall" at the inside of the ditch is simply unquarried native rock. As the walls became lower, they also became more vulnerable to assault.

Worse yet, the rounded shape that had previously been dominant for the design of turrets created "dead space" which was relatively sheltered from defending fire, because direct fire from other parts of the walls could not be shot around the curved wall. To prevent this what had previously been round or square turrets were extended into diamond shaped points which would give storming infantry no shelter. The ditches and walls channel attacking troops into carefully constructed killing grounds where defensive cannon can wreak havoc on troops attempting to storm the walls, with emplacements set so that the attacking troops have no place to shelter from the defensive fire.

A further and more subtle change was to move from a passive model of defence to an active one. The lower walls were more vulnerable to being stormed, and the protection that the earth banking provided against direct fire failed if the attackers could occupy the slope on the outside of the ditch, and mount attacking cannon there. Therefor the shape was designed to make maximum use of enfilade fire against any attackers who should reach the base of any of the walls. The indentations in the base of each point on the star was to shelter cannon located there. Those cannon would have a clear line of fire directly down the edge of the neighboring points, while their point of the star was protected by fire from the base of those points.

Thus forts evolved complex shapes that allowed defensive batteries of cannon to command interlocking fields of fire. Forward batteries commanded the slopes which defended walls deeper in the complex from direct fire. The defending cannon were not simply intended to deal with attempts to storm the walls, but to actively challenge attacking cannon, and deny them approach close enough to the fort to engage in direct fire against the vulnerable walls.

The key to the forts defense moved to the outer edge of the ditch surrounding the fort, known as the covered way, or covert way. Defenders could move relatively safely in the cover of the ditch, and could engage in active counter measures to keep control of the glacis, the open slope that lay outside the ditch, by creating defensive earthworks to deny the enemy access to the glasis and thus to firing points that could bear directly on to the walls, and by digging counter mines to intercept and disrupt attempts to mine the fort walls.

Compared to medieval fortifications, forts became both lower and larger in area, providing defence in depth, with tiers of defences that an attacker needed to overcome in order to bring cannon to bear on the inner layers of defences.

Firing emplacements for defending cannon were heavily defended from bombardment by external fire, but open towards the inside of the fort, both to diminish their usefulness to the attacker should they be overcome, but also to allow the large volumes of smoke that the defending cannon would generate to dissipate.

Fortifications of this type continued to be effective while the attackers were armed only with cannon, where the majority of the damage inflicted is caused by momentum from the impact of solid shot. While only low explosives such as blackpowder were available, explosive shells were largely ineffective against such fortifications.

The development of mortars, high explosives, and the consequent large increase in the destructive power of explosive shells and thus plunging fire rendered the intricate geometry of such fortifications irrelevant.