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In many medieval portrayals, the walls of castles are always portrayed as square. But would the castle be better off if the walls were circular instead of square?

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    $\begingroup$ There are advantages to both designs (circle vs square). In reality, castles are compromises between defence and domestic practicality, and often include both types of wall. $\endgroup$ – Steve Bird Aug 29 '15 at 22:59
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    $\begingroup$ @JohnWDailey - a sphere is a ball shape. I think you mean a cylinder. Many castle walls, towers and turrets take the form of a cylinder, e.g. feelmadrid.com/manzanarescastle.html $\endgroup$ – chasly from UK Sep 1 '15 at 9:56
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Each has its advantages and disadvantages. Circular walls are more likely to deflect cannonballs with minimal damage, but the outermost point of a circular wall is hard to defend. Interlocking fields of fire are all but impossible in this situation.

Square walls are more prone to suffering damage from cannon fire, but it is easier to generate interlocking fields of fire to defend them.

In the early medieval period, cannons weren't a problem, because at first, they didn't exist, and then they existed, but were unreliable and lacked the power needed to knock holes in thick stone walls.

As cannons improved, so did walls. They became rounded or angled to deflect big projectiles while absorbing minimal damage.

In the late medieval period, the firepower produced by cannons was too much for any walls to withstand for any length of time, and the pattern became low profile walls with gently sloping earthen ramparts to absorb cannon fire.

So the best design for your castle depends on how big of a problem cannons represent. As cannons become more powerful and numerous, walls should first be rounded, then lowered and protected by hills of loose soil.

If cannons aren't an issue at all, or if your walls are low to mitigate the threat posed by cannon fire, your walls should be angled to allow your defenders to create interlocking, overlapping fields of fire. The more men you can get to fire over the same patch of ground, the safer you'll be.

Square Walls:

Fields of fire square
Source
Field of fire of a single cannon on a rectangular tower

Rounded Walls:

Round Tower Dead Zones

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

Field of fire Round tower
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Fields of fire of a single cannon on a circular tower

Star Fort:

Fields of fire Star Fort
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The ideal solution: Overlapping fields of fire, only possible with a star fort.

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    $\begingroup$ Great images! (A little joke I can't help but share), the first one with "Dead Zones" would be more accurately portrayed as "Live Zones" ;) $\endgroup$ – DoubleDouble Sep 2 '15 at 17:23
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Yes, circular towers are better.

  1. They are more robust and harder to break by trebuchet or cannonball. And they are less likely to crumble if someone try to attack the foundations of the towers.
  2. The defenders can attack from every angle since there is no corner.

On the long term, if you want your fortresses to withstand cannon fire, you're better with triangles. It's from the renaissance but nothing prevents it to be built earlier. The style of fortification is named Vauban after the french architect/engineer Sébastien Le Prestre de Vauban.

  • The use of advanced bastions is good for protection since the soldiers can harass the attackers from above while they are trying to reach the main fortress.
  • The bastions are also meant to block direct hits against the walls by serving as an obstacle in the cannons' line of sight. Indirect hits inflicts less damage.
  • These fortresses generally had lower but thick walls and were shaped like stars to make it harder to fire shots perpendicularly.
  • The result is that they are much harder to destroy.

Examples:

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(source: ikonet.com)

enter image description here

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  • $\begingroup$ What are the fortifications in the pictures? $\endgroup$ – HDE 226868 Aug 30 '15 at 1:42
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    $\begingroup$ @HDE226868 The second one is from Neuf-Brisach in France. $\endgroup$ – Vincent Aug 30 '15 at 1:49
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    $\begingroup$ Vauban is not really medieval and his forts had to deal with advanced gunpowder technology. They were mostly built on the coast to fire at boats. The walls do not really resist to canonball individually but several remparts make it difficult to reach the center, or to destroy every military part. $\endgroup$ – Tyrabel Aug 30 '15 at 11:30
  • $\begingroup$ @Tyrabel : "Vauban is not really medieval and his forts had to deal with advanced gunpowder technology" I mentioned it in the answer already. $\endgroup$ – Vincent Aug 30 '15 at 14:19
  • $\begingroup$ "They were mostly built on the coast to fire at boats." Nope: sites-vauban.org/The-Vauban-fortifications-in $\endgroup$ – Vincent Aug 30 '15 at 14:22
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There are many reasons:

Stones

It's more difficult to sculpt semi-circular stones than straight stones. In order to have a perfectly round wall/tower, each stone needs to have the perfect shape, not a big square. That's a lot of work!

Inside

In a square room, furniture are easier to build. Along a straight wall, you can easily make a corridor. In medieval times, the castle was very often a place where a lot of people were living. Workers were building theirs workshop against the wall. It's easier to build something new against a straight wall than a round one.

But? The round towers?

Yes, there ARE some round walls and tower. In fact there is not much round walls in the X-XI centuries, but more and more with time until the XV century where there are almost every time round towers. The apparition of canonball changed the architecture.

Arrows (and stones launched by a trebuchet) are easily stopped by a wall (square or round), but canonball are not so well stopped at all, so they began to build round towers to make canonballs slip on the wall instead of breaking it.

Carcassonne city, IX to XIII century (France):

Salses Castle, anti-canonball, XV century (France too, what a lovely country for castle lovers):

enter image description here See? Old castle: little round towers against arrows and stones, big straight walls.

More recent castle: big round towers against cannonballs.

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    $\begingroup$ Actually round towers were adopted because they were more resistant to undermining than square towers. Also once gunpowder artillery became common and reliable (circa 16th Century), fortress design changed completely, with lots of angled walls to give overlapping fields of fire (see star forts). $\endgroup$ – Steve Bird Aug 29 '15 at 23:27
  • $\begingroup$ Salses castle is one of the first and last of his kind. It's conceived against canonball, it's in a hole and not on a mountain like other castles in the area, but did not go through a lot of battle, because of its history (at the end of its construction, it was no more in a conflict area, and gunpowder technology became so powerfull so fast that it loses its purpose) $\endgroup$ – Tyrabel Aug 29 '15 at 23:40
  • $\begingroup$ Also... I'd like to point out Carcassone, that is an awesome game and apparently a French castle :P $\endgroup$ – HadesHerald Aug 30 '15 at 2:38
  • $\begingroup$ Carcassonne is a french city/fortress, and there is a fabulous castle inside the fortress. It's one of the biggest and best preserved medieval fortress city (maybe the biggest) in europa. Its fortification began with the roman empire, but the towers and second exterior wall on the picture are medieval, and have been restored by Viollet le Duc in the XIX century. It has never fall to the ennemy in a battle, but had difficult times with sieges. (And its beautiful and impressive fortifications did inspire the game :P ). $\endgroup$ – Tyrabel Aug 30 '15 at 11:25
  • $\begingroup$ Note that large trebuchets could knock most walls down. There have been cases of cities and castles surrendering when they saw the besiegers had a trebuchet, because they knew the siege was pretty much over as soon as it started lobbing rocks. However, they were large, expensive, cumbersome and required trained siege engineers to build and to assemble. Cannons later made the same power accessible to a much larger scale, and in much smaller individual packages. $\endgroup$ – Eth Jul 16 at 15:42
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Sometimes castles are about status and economic power, where defence takes a secondary role

As others have correctly stated, castles with round towers (until the invention and widespread use of gunpowder) are more defensible and subject to less undermining in a military situation.

However, castles are not just for defence and/or military purposes - if you're not on the 'front line' of defence, many castles in the world serve as administrative centres, political statements or signs of affluence.

In a cursory top 25 most-beautiful castles in the world list, the majority all have square towers, likely as these are easier to furnish, more functional in terms of accommodation, and afford a pleasant outlook. The resultant square rooms are also easily extendible, easier to roof and simply more comfortable to be in.

The following are a small sample of square towered castles:

  • Eltz Castle, Germany
  • Eilean Donan, Scotland
  • Edinburgh Castle Scotland - a good example of an already defensible castle being perched on an outcrop, so the priority for the castle can be utility and comfort enter image description here

  • Bran Castle Romania

  • Kilkenny Castle Ireland
  • Mont-Saint-Michel, France - a good example of a square towered form inside an easily defensible round-towered base, in an already isolated island. enter image description here

  • Windsor Castle, England - a good example of a castle situated in a city - so at low risk of being assaulted. This castle is more about status and amenity. enter image description here

  • Alhambra Spain - very luxurious and more about status and projection of power enter image description here

  • Vianden Castle, Luxembourg

  • Bojnice Castle, Slovakia - obviously luxurious and picturesque and a symbol of power and strength. enter image description here

  • Doorwerth Castle, Netherlands

A common thread is the square castles are built for comfort, in already prominent positions that allows them to be furnished and comfortable, roofed for amenity, and located already securely either within practical city walls perhaps with round towers that take the role of utility and military defence, or in an easily defensible position already (such as within a city like Windsor Castle), and where square rooms allow more comfort and better outlook.

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