The location of the Monastery on a strategically important road led to its inclusion in the defensive system around Moscow with the construction of fortifications in 1540-60. The original wooden palisade was replaced by stone walls sorne 1.5 km long, with 12 towers. The area of the Monastery was extended at this time, and new buildings were erected, including accommodation for the Tsar, a small church over the tomb of Nikon, an infirmary and cellarer's chambers, and, most important of all, the large Cathedral of the Assumption (1559-85). The villages and settlements around the Monastery expanded to house the many workers involved in the new works.