Why and how did late-medieval/early modern era militaries start building star-shaped forts? How were they used to defend against armies with gunpowdered weapons?

Man, I can't even fathom how the producers of the video got to an idea like that. And this is the second time within a week I see a question referring to said video, and I do so hope it's the same one because the thought there are several version out there... shudders

The star fort is a direct descendent and rational evolution of fortifications in the face of development of military technology. I should note here that am going to try and go linearly here for simplicity's sake. The forces at work don't neatly move from one step to another and often parallel developments happen. Also some terms and such overlap. And am going to try and avoid the "jargon", various parts of fortifications have fancy names to them not really important in the context.

We'll start in the middle ages when high walls were mostly enough to keep people out. These walls are more high than thick. While it's possible to batter your way through it's often not easily nor practicably done. One of the key issues is being able to accurately hit the same point repeatedly. Deploying siege artillery powerful enough is exceedingly resource intensive. Even then storming a breach is not easily done in the face of the prepared extra defences likely added. The defenders are preparing for just such a scenario and they'll know exactly where you are going for. It's bloody and unpredictable. Especially if you plan on strategic conquest you do not want a fortified place stormed as the likelihood of damaging it is very high. It'll take most likely weeks to bombard a wall with non-gunpowder artillery, if not months.

This changes with the introduction of more and bigger cannon. But it is a gradual process. In 1415 the English besieged Harfleur from 18 Aug to 23 Sept, bombarding it with torsion and gunpowder artillery. The French retake it 8 Dec 1449 to Jan 1 1450. I want to say it was one of those examples where a place is slowly besieged using older techniques but quickly falls to more advanced siegecraft, I remember such an example from the English-French conflicts, but can't find it exactly now. It's possible my memory slightly fails me and the retaking happens further into the gunpowder era. I really wish I could recall it now.

The need to improve defences against gunpowder artillery becomes more acute when the Italian Wars start as the French kings decide to expand their influence amongst the principalities of Italy. The French have considerably expanded and standardized (relatively speaking) their artillery so it effectively becomes an arm to go alongside the previous infantry and cavalry. This matters because the Italian cities are rich and don't want to be looted by the French. So they invest money and human resources into figuring out a solution. The first attempt is to lower and thicken the walls. Often older castles are re-built with "artillery towers" round relatively low structures with very thick walls. In Sweden several such examples survive from the 1500s as the castles were shortly after the upgrades relegated to secondary military importance as the frontiers quickly moved beyond the Baltic in the 1600s. In continental Europe the constant conflicts mean many older fortifications are effectively torn down and replaced as military necessity demands. The English Tudors also built a lot of so called Device Forts which show the style well, Deal Castle being an excellent example.

The next step is to increase the amount of artillery not only inside, but also on top of the walls leading us into the trace italienne style of the 1500s*.* Or the Italian style fortification. Not necessarily because it was "invented" there, but due to the Italian Wars the need and motivation to rebuild fortifications existed here at the time and in an abundance. In English they are called bastion forts. They consists of low, thick walls, tending to be sloped with space for artillery and gunners at the top. To further reinforce this type we add sloped earthen ramparts in front of them to stop cannon balls hitting the base of the walls. Between the rampart and wall we have a moat to stop the attacker just easily running up the slope to the top of our wall. These bastion forts already tend towards the star shapes. The reason for this is to provide mutually supporting and overlapping fields of fire. Which ever face you try and attack even though you affect a breach you are likely subject to withering crossfire by gun and cannon if you storm it. The earlier round bastions would have had dead zones directly in front of the tower which you avoid by using a pointed bastion. Each face of the bastion has fields of fire it covers. Over time the design becomes more elaborate to thicken the defensive perimeter and cover any dead space that may exist in the design. And not all fortifications were created equally. Various designs and ideas are applied throughout.

Effectively star forts is a development and formalisation of the bastion fort style using geometrics and mathematics to create "ideal" arcs of fire and a defence in depth. It'd probably be remiss not to point out that the Renaissance times like symmetry a lot, whether it impacted garden design, city plans or fortifications so the style does not only cover an actual military need it caters to the periods taste (never underestimate the sue of being able to make a cool looking plan to the higher-ups). Speaking of the ideal, I should point out here that theory and practice would usually have to compromise. There are very few cases where a completely new star fort was built where no consideration had to be taken to terrain or existing structures. Normally you already had something to start with, an earlier fortification or town. After all, most of these are built to protect existing assets. The classic examples are from the 1600s and 1700s and often attributed to Vauban (he didn't oversee the construction of all so named), who was the chief fortification engineer of Louis the XIV. In a French context therefore often known as Vauban fortresses, and arguably the man is responsible for our general picture of a star fort (whether we are cooks or not). In his time one of the most celebrated military engineers, he introduces a standardization of the siege process, and in fact would pride himself to being able to accurate calculate how long a particular siege would take even before it began. Yes he didn't just design fortifications, he also figured out how to defeat them. Which of course in turn informed his ideas of how to construct them...

At the height of their construction a star fort could be an extremely complex structure. What we seen on a general top-down plan is a lovely symmetrical structure. But normally such depictions do not shows us the ground-level cross-section with all it's various glacis, ramparts, firing-steps, moats, counter-scarps etc. etc. forming a roughly triangular shape rising from ground level up to the top of the parapet of the innermost bastion. Every step of the way was supposed to be covered from the flanks and from above by another layer of firing arcs.

As history rolls on artillery ranges increase and the last examples include outlying supporting starforts to stop long range artillery from hitting the main one. In the latter parts of the 1800s the starforts are effectively obsoleted by developments in artillery, among them exploding shells that make short work of the earth ramparts. And fortifications go into a rapid process of development and counter-development where forts are often obsolete as soon as finished, eventually ending up with our modern concrete reinforced bunkers. But that's another question entirely, and probably also full of aliens if history channel and the online cooks have their way.

Some words about assaulting such fortifications. As mentioned above Vauban also revolutionised siegecraft, sort of. The bastion style and starforts after all are a reaction to how they are attacked. The original medieval castles are still besieged quite close by, even as cannon come along not necessarily out of bowshot even. As fortifications increase the number of cannon and gun armed soldiers on the wall besiegers have to start digging down. Now mining of course has a long history in siegecraft and one of the best ways to breach fortifications was to dig under them. As the bastion style develops attacking armies have to start digging trenches to protect themselves and their own cannon. These trenches have to zig-zag towards the fortifications because you do not want the enemy to be able to enfilade you right down your trench. Effectively you trace a line parallel to the enemy works. The weakest point of the bastion/star forts is the point itself as this is furthest away from the guns covering it. So a formal siege would roughly start by surrounding the enemy fort with trenchworks of it's own, then you dig a battery positing that can start bombarding the point of a bastion. It's this digging work that enabled Vauban to predict the length of sieges. Ultimately forts cannot stand forever, and the defender knows this. It wasn't uncommon that siege operations would progress to a point where a breach is opened in a bastion, whereupon the defenders are able to surrender honourably. The end is not really in question and the people at the battle don't have to get themselves killed. Obviously if you didn't surrender at this point you better hope someone higher up is coming to rescue you because if they have to storm your fort the enemy are going to be angry. This may seem somewhat odd from our more, I want to say more fervour position. But people back then had less reason to die for another man's cause. It's an old tradition really. But also fundamentally fortifications were generally not expected to hold indefinitely, they were there to slow down, wear out and generally annoy the attacker so the defenders had time to raise their armies and relieve the fortress.