Why were fortresses often built pentagonal?
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It’s the fewest number of sides where the defense weapons of the time (cannon) could have overlapping fields of fire. Cannons on both walls can target the same attackers.
A 4-sided fort means each side is facing a completely different direction so they can’t do that.
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Any raised position have a disadvantage if the enemy is able to get close enough. The same fortifications that protects you from incoming shots from further away is now stopping you from firing at the enemy. This is true for both crossbows, cannons and muskets. There were a few solutions to this such as moats but they were not perfect. One of these solutions was to build the fortress such that there are towers that can shoot the outside of any wall. So even though the defenders on the walls can not fire at the enemies directly under them a tower further away can. But if you build a square fortress you still have blind spots. An enemy can approach a tower on a corner and use the tower itself to hide from the other towers as well as the walls. But if you make a pentagonal fortress then there are very little room to hide in this spot. The towers can all protect each other fully.
The extreme end of this trend is the “star fort” – you can see loads of these across the low countries and france. As others have said, its because of the huge influence cannons and howitzers had on siege warface at the end of the medieval period.
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If we’re talking about bastion forts (which I guess, because pretty most pentagonal fortresses are bastion forts), aka star fortresses then pretty much everyone in this thread is wrong.
a) There were quite a few 4 sided bastion fortifications (like for example [Svartholm](https://upload.wikimedia.org/wikipedia/commons/9/94/Svartholma_from_air.jpg) in Finland). So you could totally build a 4 point bastion fort.
b) It’s not about supporting fire, because bastion forts with 4 points are just as capable of supporting each bastion as a 5 sided bastion fort (as long as all of the angles were correct). It’s about interior volume. Each bastion is a fairly exact distance from each other bastion where musket fire from one bastion is able to lay down effective enfilading fire to protect its neighbours (enfilading fire is guns firing along the side of the wall so that each bullet has the maximum hit chance if a whole bunch of soldiers are trying to scale the bastion). Then each bastion is joined by a curtain wall. So with the size of each bastion and curtain wall limited by how far supporting fire from neighbouring bastions could reach you had to create more bastions to get more interior space.
A very small bastion fort had 4 bastions joined by 4 curtain walls (although it probably had an outer fortification protecting the gate that formed a 5th point, like [Castillo de San marcos](https://cdn.britannica.com/68/95668-050-81424504/Castillo-de-San-Marcos-fortification-Spanish-St.jpg)).
As bastions increased in size they got more and more points, like [this fortified city at Naarden](https://home.kpn.nl/pagklein/images/20100521a.jpg) with 6 bastions. Italian cities with a bastion-fort-like outer wall could have dozens of bastions. Note the gun battery emplacements behind the “arrowhead” of each wall. Placing a gun battery in this position gave them a very useful field of fire (firing down the side of the wall) but putting them low and behind the high walls of the bastion arrowhead also meant that they could only receive return fire from a very narrow angle. So it was hard for the attacker to direct firepower at these gun batteries, while anyone trying to storm the fort would have to expose themselves to the defenders gunfire at close range (where they could fire grapeshot/canister shot. Which is basically turning a cannon into a big shotgun).
the design concept does not revolve as much about the number of sides, but general arrangement of such fortresses (towns). It was also a great leap forward in terms of organization compared to overpopulated medieval towns which were ideal for spread of rebellion’s, plagues and diseases due to lack of planning or organization
[https://www.archdaily.com/974799/exploring-the-history-of-the-ideal-renaissance-cities](https://www.archdaily.com/974799/exploring-the-history-of-the-ideal-renaissance-cities)
the concept of an “ideal renaissance town” was first proposed in 15th century, and has had numerous iterations by many famous architects of the era.
Importance of the design was not only that it defended from the invaders on the outside, but it also defended from the “core”, as a small platoon based in the center of such town could cover all the approaching alleys in case the outer perimeter is breached, or in case there is a rebelion within the walls.
Another important aspect was that designers often incorporated water ways as a method of connecting the city center with the perimeter, something that would be useful even for hygienic purposes.
it also gave way for Leonardo da Vinci’s multi layer city concept , where the streets/waterways were on different levels, where you’d have ship traffic one one level, normal daily trades on second, and pedestrian “posh” ways on a separate level.
[https://phys.org/news/2019-05-leonardo-da-vinci-ideal-city.html](https://phys.org/news/2019-05-leonardo-da-vinci-ideal-city.html)
one of the earliest designers for military purposes was Sangallo,
[https://www.agriturismosomaia.it/en/medicea-fortress-of-giuliano-da-sangallo-sansepolcro/](https://www.agriturismosomaia.it/en/medicea-fortress-of-giuliano-da-sangallo-sansepolcro/)
but more on the topic in general:
[https://www.sgira.org/pub13.htm#:~:text=Among%20the%20most%20important%20architects,Sangallo%20family%2C%20and%20Michele%20Sanmicheli](https://www.sgira.org/pub13.htm#:~:text=Among%20the%20most%20important%20architects,Sangallo%20family%2C%20and%20Michele%20Sanmicheli).
It has to do with the angles of the corners as well as the interior space of the fortress. In the gunpowder age fortresses were protected by cannons stationed in what were called bastions. Bastions were so shaped to allow the guns to shoot along the walls of the bastions and between the bastions. The principle was that every inch of the fortress’s wall was covered by guns.
The fewer number of sides to the fortress, the narrower the angle at the salient (tip) of the bastion. This meant it was weaker against enemy gunfire and had less interior space, so the guns could be more crowded and harder to work. Also, there would be less total space inside the fortress itself, so there would be less room for barracks, magazines, storehouses, etc.
On the other hand, the more sides the fortress had the more guns it needed for defense to cover all those walls. So that was a design consideration too.
Smaller earthwork forts were often square, but for a masonry fort five sides (and five bastions) was usually the minimum. The pentagon had a good balance between interior space and number of guns need for defense. A large fortress built around a city would usually have many more sides, sometimes as many as one or two dozen.
Once in a while someone would build a triangular fort but that was very rare. They were just too small and weak.
Every wall is mutually supported by two other walls. So, an attacker will face 60% of the fort’s firepower regardless of where they attack. With a four-sided fortress, the attacker would only have to contend with 25% of the fort’s fire power.
Cannon mounts can turn 130ish degrees in to either side so around a 260 degree line of fire. When you have a pentagon you have an internal 108 degrees and an external 252 degrees so by having the fort in a pentagonal shape you allowed more cannons to shoot at a certain target because of the overlap.
Lets say you got enough materials to build fort walls that make a perimeter of 100 meters and it is a square shape. The cannons on the wall can shoot forwards and to the side by 130 degrees so if the enemy is coming from the left the cannons on the front, back and right can’t shoot him. Now if you had the same perimeter built but in an pentagon shape it’s a different issue because now your cannons from two sides can support the side that is being attacked. So for simplicity’s sake if the cannons have to be spaces 5 meters apart in a 100 perimeter you’d have 25 cannons pointing to one side if it is a square but it if it was a pentagon you’d have 20 cannons pointing to one side and around 6 from two other sides to support it so 32 cannons can attack the enemy.
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