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In my setting there is a tidal island fortress similar to Mont-Saint-Michel that is the refuge of important nobility and is therefore a target for an invading army. The fortress also has a library containing an extremely valuable manuscript that the invading king absolutely must have. So there is motivation for besieging such a place.

The Defenses

The tide is strong and makes the fortress an island during high-tide and leaves behind a treacherous mudflat at low tide that is very difficult for men to walk on and is impossible for any cart or horse to cross. The water is too shallow for large ships, but shallow draft boats and rafts can cross.

A talus wall and glacis encircle the island and provide resistance to cannon fire. There is a secondary wall further up the island and a castle keep.

Defenders Resources

The Island has freshwater wells and several large gardens along with significant food storage space. The defenders have a well trained garrison of 200 men equipped with crossbows, polearms, swords, matchlocks, cannons and even a primitive volley gun

Attackers

The attackers are several thousand strong and at a 14th century European level, and can do any tactic that you believe would work. No magic allowed

So how could an army with 14th century technology capture a well defended tidal island?

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    $\begingroup$ Minor quibble: Possibly my understanding of geology and water science is limited, but how does a tidal island that low and small have freshwater wells? Dig down on an island that small and I do not believe you will get fresh ground water. I would be more inclined to believe in large cisterns supplied with carefully collected rainwater. $\endgroup$
    – KerrAvon2055
    Mar 1, 2020 at 6:01
  • $\begingroup$ @KerrAvon2055 It’s not uncommon for tiny oceanic islands to have freshwater. It’s called a lens but rain collection works as well $\endgroup$
    – user71781
    Mar 1, 2020 at 6:25
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    $\begingroup$ i assume surround and wait till their supply is depleted is out of the option ? thats the usual way to capture castle after all. $\endgroup$
    – Li Jun
    Mar 1, 2020 at 6:35
  • $\begingroup$ @Li Jun That is the sensible option $\endgroup$
    – user71781
    Mar 1, 2020 at 6:37
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    $\begingroup$ Two words: Trojan Seahorse. $\endgroup$
    – user535733
    Mar 1, 2020 at 16:11

3 Answers 3

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Dispute the mastery of the Empire.

Sun Tzu lays it out

  1. The rule is, not to besiege walled cities if it can possibly be avoided. The preparation of mantlets, movable shelters, and various implements of war, will take up three whole months; and the piling up of mounds over against the walls will take three months more.

  2. The general, unable to control his irritation, will launch his men to the assault like swarming ants, with the result that one-third of his men are slain, while the town still remains untaken. Such are the disastrous effects of a siege.

  3. Therefore the skillful leader subdues the enemy's troops without any fighting; he captures their cities without laying siege to them; he overthrows their kingdom without lengthy operations in the field.

  4. With his forces intact he will dispute the mastery of the Empire, and thus, without losing a man, his triumph will be complete. This is the method of attacking by stratagem.

You do not attack the island fortress. You capture (or threaten, or don't) the rest of the territory and wait. The nobility are not nobility of this island. They have holdings on the mainland. Probably they do not know what is going on there. Probably they are worried and would like to go home, where the servant to nobility ratio is more favorable.

Tell them your version of what is going on. Send an emissary to the island. The emissary will explain that his master (you) wants the contents of the library and select other items from the treasury. He would rather not anyone get hurt. If a noble would like to bring some of the described items out, his master will show his appreciation by allowing that noble(s) safe return to his lands, which will not be attacked. After 3 days, any nobles still in the refuge will have their lands attacked and confiscated. Nobles still in the refuge may remain there.

The nobles want their lands because that is their wealth. They have no interest in the stuff in this refuge which is not even theirs. Why shouldn't they hand it over for safe passage home?

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    $\begingroup$ Sun Tzu is the man $\endgroup$
    – user71781
    Mar 2, 2020 at 2:37
  • $\begingroup$ Won't work as described if the defenders are a kind of knighty monks. You'll need to find different incentives. $\endgroup$
    – Adrian Colomitchi
    Mar 2, 2020 at 9:53
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Biological weapons

Anthrax infections from direct contact are rare, however infections by contact with blood/organs of an infected organism are likely.
So, procure anthrax infected sheep and catapult them over the wall to make a big splat on the other side. Keep doing it for a while.

Add brucellosis - not fatal but wasting, also require contact with the fluids from infected animals.

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  • $\begingroup$ Biological warfare is definitely a sound choice. $\endgroup$
    – user71781
    Mar 1, 2020 at 6:26
  • $\begingroup$ you can also throw poop or corpse to create epidemic even better if the poop manage to land on their water source $\endgroup$
    – Li Jun
    Mar 1, 2020 at 6:30
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    $\begingroup$ @LiJun oh, let's be civilized, shall we? I mean, once they die (their fault anyway, why did they opposed me - mwha-ha-ha-ha) I may want to use that water. At least if it's clean, I can boil it. $\endgroup$
    – Adrian Colomitchi
    Mar 1, 2020 at 6:35
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Two solutions: long one, depending of the qualifications of the attacking army, of strategic situation and of how you much is "several thousands", is to build a reinforced building from the coast to the island. This building could a floating bridge or an actual bridge, only built while the tide is low.

Short one could be to use some specific weapons or siege technics, like exploding powder, to destroy part of the walls in a short time. Then, before the defenders could reinforce the breach (for example at next tide), the attacking army rush in and fight until the death of 200 defenders. With a ten to one or more ratio, it would be an easy task.

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    $\begingroup$ Pontoon bridges are a tried and true tactic $\endgroup$
    – user71781
    Mar 1, 2020 at 9:25
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    $\begingroup$ Right, I was looking for the word. But on dangerous sands, I am not sure if it easier to build than a floating bridge $\endgroup$
    – totalMongot
    Mar 1, 2020 at 9:29
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    $\begingroup$ The problem I see is, that any bridge will be ripped apart by the defenders cannons as soon as it comes into range. $\endgroup$
    – Sebastian
    Mar 1, 2020 at 19:34
  • $\begingroup$ Depends of the precision and the strength of the cannons $\endgroup$
    – totalMongot
    Mar 1, 2020 at 19:59

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