All of which was actually somewhat interesting to a Princess who had learnt geometry sitting on the knee of Baron Gottfried Wilhelm von Leibniz. But artillery got better very gradually, and gunners now knew all the math they would ever know, and so none of this had changed much during the decade or so that Caroline had been passing through it almost every day. Riding among the fortifications was a time to brood or to day-dream. Her senses did not engage the world until she was crossing over the second of two causeways, strung across an inundated waste, put there to keep Louis XIV’s guns at a decent remove. The extremity of the fortifications was a timber gatehouse at the place where the planking of the causeway turned into gravel.