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In the article Een officier en een gentleman: Willem Norwood en het beleg van Maastricht, (An Officer and a Gentleman: William Norwood and the siege of Maastricht), on page 7 it mentions concern about the French spying on the city before the expected attack.

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Translation: Watchwords and passwords

"On May 12, (1672) Norwood wrote down (in his notebook) the passwords used to identify enemy agents, such as French spies. Apparently people were afraid that French spies would enter the fortress. Every day at six o'clock in the morning, the adjutant received the orders and passwords at the main guard on the Vrijthof (Square). And through the officers on duty, these passwords were further distributed among the guard posts and patrols in the fortifications."

What specifically would the spies be looking for? I am most interested in the specifics of the strengths and weaknesses of the fortifications that the spies would investigate, but any other information about what military spies would be doing would also help.

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    Likely the usual stuff that spies do: how many defenders are there, how much food, weak places in the defenses, what kind of weapons they have, etc.
    – Roger V.
    Commented May 17 at 15:20

4 Answers 4

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Supplies, Morale, Weaknesses

Before a siege a spy would be most interested in the following information:

Food/Water supply: This determines how long a city can withstand a siege. Remember that, historically, most sieges were not "get through the walls somehow and have a big fight until the defenders are dead/surrender." They were "sit outside and put pressure on the garrison until they surrender/starve." However many sieges were ended because the ATTACKERS ran out of food. The countryside can only support an army for so long, and then the tyranny of the wagon equation takes hold. If the attackers know that the defenders can hold out for X weeks, then they know they have to be able to keep their army supplied for X weeks + however long. If the attackers can't do it, then their options are more limited. The amount of supplies also can determine how much support can help the defenders. If an attacking army besieges a city, and that city only has food supplies for 3 weeks, a relief army 6 weeks away isn't something the attackers have to worry about.

Morale: Are the defending troops willing to defend the walls? Are the citizens supportive of that act? Will the city leaders beg the garrison to surrender, or will the citizenry take up arms to bolster the garrison? In a period where a city that fought its besiegers would suffer rape and pillage by the attacking troops as a matter of course morale was critical. Knowing the morale of the troops, civilians, and leaders of both would be pivotal in crafting a suitable plan of attack.

Weaknesses: A place in the wall where previous attackers have come through and was badly repaired, a section of the garrison that's poorly trained/led and susceptible to surprise, those city elders who might be bribed into convincing the garrison to open the gates instead of attack, a poorly positioned fort that's exposed to attack from a certain angle... more esoteric and, in my opinion, less frequently critical to the attackers success, these things could still be important factors. Knowing a city was suffering from a disease outbreak or just had half it's garrison sent off to some other location is always good information to know!

I've listed them in order of general importance, but specific circumstance could render one or the other more/less important. At Ciudad Rodrigo, for example, Wellington needed to take the city quickly because of outside factors, so any spy would likely have been told to focus purely on weaknesses in the defenses/garrison since there was no time for a protracted siege.

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    Thank you. The issue of the weakness of the fortifications is most important as I am studying the 1673 French siege of Maastricht. With Vauban leading the siege and applying parallel trench tactics for the first time, the French had no intention of starving Maastricht into surrender. Their goal was the breach the walls of the city, in a matter of weeks, if the leaders of the city didn't surrender first.
    – Bob516
    Commented May 17 at 19:29
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Spies collect all kind of information. They gather data, and, for the best of them, separate the wheat from the chaff, in order to transmit only the best information.

From Les espions au Moyen Âge (spies during the middle ages - FR only):

Il lui faut connaître les commandants, les coutumes locales, les habitudes des camps, s'adapter à elles pour être crédible dans son rôle. Il doit être familier des équipements, des infrastructures et de la topographie des lieux.

He must know the commanders, the local customs, the habits of the camps, and adapt to them to be credible in his role. [...] He must be familiar with the equipment, infrastructure and topography of the place.

Spies needs to pinpoint weaknesses and forces of the opponent. The inner structures of castle/cities offer excellent information for the assailant. The number of combattants, civilians, the type and number of weapons. The amount of food/water. Strengths and weaknesses of infrastructures help tactical choices too.

For instance, an ancient door/gate to some part of the city. Once you modify the structure, say, to strengthen one side, you need to fill the gap with new stones and sealing cement. Nevertheless, this creates a weakness in the structure. A spy discovering that part of the wall would be of huge help for the assailant.

If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. Sun Tzu - the art of war

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To further the answers of others a spy could not only be an observer but an actor, e.g. find out the stocks of grains, water and morale, then sabotage through fire/poison/disease, maybe even murder someone important or take out the communication network by killing/freeing pigeons etc. to prevent allies of the besieged from arriving.

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    Sources would improve this answer. Thank you. Commented May 20 at 4:45
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The typical concern with infiltrators during a siege is that they will open the gates, giving the besieging army an easy entrance to the fortified area without having to fight so much.1

This is such a classic concern that it was how the Greeks ended the Trojan War, according to Homer. Of course the Odyssey isn't a historical document, but it does show how supreme of a danger people of the ancient era considered opening the gates to an invader to be.

Another thing spies could do to end a siege is poison the fortified area's water supply. Typically one or more wells would be the target (if the city wasn't using a local water supply, the deed could be done without needing spies.)


1 - Sadly, doing this is a game mechanic in the very popular Medieval: Total War, which is producing so many hits for this on a web search I'm having trouble finding anything else, like say an actual historical incident of it happening.

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    I am looking for information about what spies would do before a siege began.
    – Bob516
    Commented May 17 at 14:36
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    You mean like scoping out the water supply, the gate mechanism, and generally embedding themselves in the city for the moment where they might need to act against those things? Its a lot easier to get someone inside the walls before the gates are closed.
    – T.E.D.
    Commented May 17 at 14:42
  • Other than investigating the strengths and weaknesses of the fortifications, as I mentioned in the question, I didn't have anything else specific in mind. Since you mentioned you found no evidence of a historical incident where spies opened the gates during the siege it is not relevant to me
    – Bob516
    Commented May 17 at 14:57
  • @Bob516 "people were afraid that French spies" is more related to stories about spies than to evidence of historical incident, so you may want to change your original post if you aren't interested in what makes people afraid of spies.
    – david
    Commented May 17 at 21:15
  • At the siege of Antioch, Bohemond got the gates opened by a guard. It was a "spy" not a "diplomat" who negotiated the deal.
    – david
    Commented May 17 at 21:17

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