Nullus constabularius distringat aliquem militem ad dandum denarios pro custodia castri, si facere voluerit custodiam illam in propria persona sua, vel per alium probum hominem, si ipse eam facere non possit propter rationabilem causam; et si nos duxerimus vel miserimus eum in exercitum, erit quietus de custodia, secundum quantitatem temporis quo per nos fuerit in exercitu.
No constable is to distrain any knight to give money instead of performing castle-guard, if he is willing to perform that guard in person, or, if he is unable to do it for a satisfactory reason, through another reliable man. And if we have led or sent him in the army, he is to be quit of castle-guard in proportion to the time he is in the army at our behest.
Like its predecessor, Clause 29 was concerned with abuses originating in castles, and above all royal ones (it was capable of being applied to baronial fortresses as well, but that was probably not its primary purpose). Castle-guard, essentially garrison duty, was one of the services which men holding lands by knight tenure might be called upon to perform for their lords. It was by no means universal (it was never used to man the Tower of London, for instance), and by the early thirteenth century it had become usual for it to be commuted for cash, enabling those to whom the service was due to hire mercenaries instead. The rates of commutation were probably increased sharply in the years leading up to Magna Carta, as the king strengthened his castles and reinforced their garrisons. Stipulating that men willing to perform castle-guard in person should be allowed to do so curbed a method of extorting money, and it also gave the tenants concerned a potential role in the manning of royal castles which could have resulted in their being able to control them, or at least to reduce their trustworthiness, to John’s disadvantage. Straightforward financial issues probably lay behind Clause 29’s second provision. A knight could be required to serve in the king’s army and also in his castles, and performing the one service did not exempt him from the other – no doubt he had either to hire a substitute or pay cash in commutation for the service he did not perform, at rates imposed at the whim either of the king or of the officials charged with collecting the money. After 1215 the amount of castle-guard he was called upon to provide was reduced in proportion to the time he spent with the army.
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