Siege!

You order all your men to fetch shovels and begin digging. Your goal is to tunnel under the walls of the fortress and collapse them so the armies may simply march in. Thousands of men begin digging and soon hit patches of permafrost and rock. You order them to fetch picks to shatter through this frost and rock and slowly they begin to make progress. After reaching about seven heads deep into the earth they find that they are hitting solid rock. Progress comes to almost a stand still. The earth here is virtually impenetrable, this is why this location was picked to build the fortress, and God had made the landscape itself invulnerable to any incursion. You realize that tunneling will not be a viable option. You have lost two days in this folly attempt.