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Let's assume that in this world, walled cities are more common than castles. I have built a city, one quarter of the city faces the ocean and a river flows through the city. But others want to capture my city. As a series of statistically perfect and efficient designs, what kind of wall design (both vertical and overhead) is best suited for this city?

Some information on my attackers;

  • They attack for harvest of human sacrifice Aztecs.
  • The design of their infantry weapon is similar to Japanese.
  • Their strategy is to attack, wait, create an illusion of peace, attack again, repeat until the enemy cooperates.
  • They have the same technology level as the Europeans in early 1300's.
  • They have siege towers, battering rams, cannons, trebuchet, catapults and any other siege weapons you can think of.

All Culturally Correct Questions

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    $\begingroup$ Have you done any research on this topic? One thing I would recommend researching is the cannon. It's introduction is curiously coincident with the phasing out of walled cities because nobody could develop a wall that could be effective against the cannon. $\endgroup$ – Cort Ammon Aug 31 '15 at 17:18
  • $\begingroup$ But some walls work better than others. That's the whole point of the question! $\endgroup$ – TrEs-2b Aug 31 '15 at 17:19
  • $\begingroup$ There weren't many walled cities but people stil built forts for a long time. Usually they used materials with give, like sand, to absorb the shock without cracking. $\endgroup$ – Vulcronos Aug 31 '15 at 17:20
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    $\begingroup$ Could you precise the meaning of "They have the same war mentality as the Aztecs. The design of their infantry is similar to Japanese. Their war strategy is on par with the vikings" ? Does it means that they conduct war to harvest for human sacrifice, have curved sabres and pillage towns until you give them land to make them stop ? $\endgroup$ – Kolaru Aug 31 '15 at 17:28
  • $\begingroup$ You got it, but I'll add it $\endgroup$ – TrEs-2b Aug 31 '15 at 17:31
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The walls in pre gunpowder age civilizations were developed mostly to prevent rapid overrunning of the defended area, shelter from enemy "fire" and provide high points for observation and to deliver counter fire against invaders.

Curtain walls will also need deep foundations so they don't fall over (and to make them more resistant to mines and battering rams), and given the construction technology of the day, they need to be thick as well (no elegant engineering solutions here). This in itself limits the idea of walled cities to very rich principalities, and even then to the Imperial capital and major trade hubs, since otherwise the resource bill will overwhelm the amount of workers and materials available.

From a technical viewpoint, curtain walls are both stronger and easier to defend when they are supported at regular intervals by towers. The towers provide the effect of buttresses for part of the wall, but also strong points for the defenders, and the ability to fire across the front of the wall to engage enemy engineering troops trying to raise ladders, or move up siege towers and battering rams. Towers also provide artillery platforms for catapults or even small trebuchets, to keep the attackers at a distance (a catapult or trebuchet at ground level must be larger to match the range of an elevated catapult that is shooting at you. Moving large pieces of siege artillery is always an issue, so the attacker will often have to build the machines on the spot, with all the issues that entails).

The spaces between the towers in European castles and walled fortifications had the alternating pattern of stones and open spaces called crenellations, which allowed bowmen and others to have shelter and fire through the open spaces. Overhanging platforms were often built as well to allow soldiers to drop rocks, hot sand, boiling water, sewage or flaming debris on enemy soldiers toiling below.

The real key to fortified cities (or castles or even modern fortifications) is the resource bill for building them, maintaining them and having sufficient manpower to man the walls, extra troops to sally forth and attack the invaders outside the walls when advantageous and enough spare materials and engineering troops inside to repair the walls. Many walled cites and forts were undermined by allowing growing populations to build right outside the walls, giving attackers covered approaches to the walls, and many forts and walled cities were easily overthrown because the walls were in disrepair, or not enough troops were garrisoned inside, or the ones inside had insufficient supplies to last a long siege.

So building the walls is only the first step.

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  • $\begingroup$ Wikipedia disagrees with you "From very early history to modern times, walls have been a near necessity for every city" en.wikipedia.org/wiki/Defensive_wall . $\endgroup$ – Taemyr Sep 1 '15 at 10:54
  • $\begingroup$ Pre-gunpowder curtain walls were thin, not thick relative to later walls. Height was needed to keep troops from climbing the walls. Gunpowder led to walls being thick to absorb cannon shot without collapsing, and thus were lower because gunpowder weapons could kill attacking troops before they could get close to the defenders. $\endgroup$ – Oldcat Sep 1 '15 at 21:07
  • $\begingroup$ Thick walls and deep foundations are needed for support. Curtain walls are "thin" relative to later artillery fortifications, but if you actually measuring the wall it can be over 10' thick in order to provide the proper amount of support. $\endgroup$ – Thucydides Sep 2 '15 at 1:06
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Instead of mainly using a wall use a large area of land that you citizens can flood when they so desire.

To make a short answer longer:

The OP city is adjacent to the ocean and has a river flowing through it. Therefore, it is likely that the city finds itself in a river delta. River deltas are often plagued by floods. However, once you tame the water it has its use in producing fertile land, allowing to transport goods easily, and as a (pre-aviation) defense mechanism.

The above link points to the Hollandic Waterline, of which the are two: the "old" line mainly protecting Amsterdam and the "new" line which protected a larger part of the Netherlands.

When you flood a large area of land it becomes difficult to march armies (soldiers on foot and men on horseback) though it, and transporting heavier things like cannons is very difficult (see google's pictures). Without good preparations your men will get sick and cold and your heavy equipment will sink in what has basically become a swamp. The water level does not have to be very high.

At key points the waterline is defended by traditional forts with earthen walls. The forts' earthen walls are a good defense against cannons. To flood the land you use a combination of techniques:

  • break dikes so that the now uncontrolled water floods into a lower areas;
  • divert the river(s) again so that they flow over the lower areas;
  • stop pumping groundwater away (in some areas of the Netherlands if people stopped pump water away, the area would become a swamp; not sure if this applies to the OP city); and,
  • Keep the cities own forces mobile and prepared for this strategy:
    • you can use ships with a low draught;
    • have information about which paths remain dry helps;
    • during winter troops on ice skates were able to surprise the enemy who thought that the defenses had failed now that they were frozen.
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  • $\begingroup$ Little short but I like this answer. $\endgroup$ – TrEs-2b Aug 31 '15 at 18:03
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    $\begingroup$ "during winter troops on ice skates" MURDER! On ice! The new Disney musical! ;) $\endgroup$ – Neil Sep 1 '15 at 14:43

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