The phrase "mighty fortress" means a very strong and secure place of protection or defense. It refers to a place that is difficult to break into and provides safety against dangers. Full definition
They were wrong, and little remains other than the ruined shell of a once mighty fortress. (wolfsgamingblog.com)
The bright orange terracotta roofs and mighty fortress walls of the city's Old Town have become one of modern Croatia's most popular tourist attractions. (raildiscoveries.com)
The Protestant reformers built consensus using music like «A Mighty Fortress.» (rachelheldevans.com)