Complex sentence: His exact flightpath is not known, nor how long he was in the air, because today's abbey is not the abbey of the eleventh century, when it was probably smaller, although the tower was probably close to the present height.
Split into simpler sentences: His exact flightpath is not known, nor how long he was in the air, because today's abbey is not the abbey of the eleventh century. The abbey was probably smaller, but the tower was probably close to the present height.