In the following sentences, how should I understand the words "yet another"?

If you have any idea, would you tell me some here?

Thanks for your help in advance~!

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Until about 1150 the fortified houses of the Anglo-Norman no bility were simple dwellings surrounded by a mound of earth and a wooden stockade. These were the motte and bailey castles: the motte was the mound and its stockade, the bailey an open court lying below and also stockaded. Both were protected, where possible, by yet another ditch filled with water, the moat. In the middle of the motte there was a wooden tower, the keep or donjon, which only became a genuine stronghold at a later date and in places where stone was readily available.