This is from a paper I wrote as an undergraduate on the defence of Cyprus against the Ottoman Turks.

Around 1550 a split occurred in the architecture community. With the advancement of military and civil architecture no longer could one person master it all let alone be able to direct an entire project. Groups were now hired to design and construct a city’s defenses while another group would be hired to construct everything else. Most of the military architects were former soldiers who had firsthand experience on what worked and what didn’t. They went for pure function and left out aesthetics. On the other hand civil architects studied a wide and varied background to combine elements in their designs (Croix 1960: 273-274).

The basic plan was further improved upon by having straight walls between all the towers instead of circular which was preferred for walled cities as it allowed more area within to build. This prevented any dead space from occurring from the curve of the wall between the towers (Croix 1960: 281). A further improvement was the radial plan within the city walls. It was theorized for awhile on how it would be done and what style. Some had straight roads from the gates to the center while others had a spiral form. The idea behind a radial plan was that a commander on a tower in the center of town could see what was going on at each of the gates during the battle and direct troops and resources where they were needed (Croix 1960: 284). The first use of the radial plan was used at Meuse in 1544. The fortress was square with triangle bastions protruding outward to protect the walls (Croix 1960: 276).

Bastions and ravelins, which were mentioned earlier, were another key feature that revolutionized anti-artillery architecture. These bastions were projections from the wall that were thick and stout to withstand cannon fire. Ravelins were similar to bastions, but instead of being projections of the wall they were their own structures sometimes connected by a short wall. With both the bastions and the ravelins the section that was perpendicular to the wall contained cannon aimed up and down the length of the wall (Croix 1960: 267). This provided protection to the wall in case a breach formed. When the enemy tried to rush the gap the cannon would let loose and obliterate the charge (Croix 1960: 266). In time it came to a point where the bastions and ravelins became virtually indestructible. The shape of the bastion and ravelins evolved over time from a triangular shape to more of a heart shaped. The leading point of the triangle was more often than not damaged and blown off. By adding some curve to the shape it helped to blunt the point and prevent much damage.

The placement of the bastion and ravelins was another problematic issue amongst military architects at the time. The distance between the bastions had to be able to cover the wall and not damage the other side. One group of architects wanted the bastion walls to be thick enough so that when they did fired along the curtain and hit the other side it was strong enough to take the impact. The more popular group preferred angled walls to the bastion that allowed the defenders to fire alongside the curtain at an angle to protect the front of the other bastion and the ditch (Croix 1960: 280).

De la Croix, Horst. 1960. Military Architecture and the Radial Plan in Sixteenth Century Italy. New York, New York.

To add to what r/Mictlantecuhtli has answered;

Star shaped fort designs were not prevalent throughout history. They were invented in Italy in the 1500s (and are sometimes called "Trace Italien"), and lasted through until WWI.

They were designed to be more defensible against cannon than the previous medieval fortifications (castles and walled towns) which had proved vulnerable to cannon.

The low and thick walls of star shaped forts resisted cannon well. The "star shape" allowed cannon (and other firearms) to defend all approaches to the fort, and particularly to fire along all the walls to destroy any enemy who got close enough to try to scale the ramparts.

Star shaped forts were a very effective defense for 400 years, so many of them were built.