sentences:  Subterranean passages also linked it to the Serravalle Bastion, which was in essence a three-storey gun tower bristling with cannon.Underneath the courtyard a cistern was installed, providing sufficient water for 1000 troops for a 20-month siege, with a huge vaulted ceiling supported by nine columns.Monaco was to remain politically vulnerable for another century and little building work took place from 1581 until 1604, during the reigns of Prince Charles II and Prince Hercule.