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The castles were various sizes. Some in the Loire Valley are huge and others are relatively small as far as castles go. If you stay in a central location in a town with its own castle, its actually pretty easy to see multiple in a short period of time. Most castles are within an hours drive from each other so 2 per day is reasonable and 3 is possible if you really push yourself. 3 wouldn't be enjoyable though because you would be too rushed.