What does a fortress symbolize in the Bible?

In our lives, we all have fortress we run to when we are in danger, in times of need etc. Most of the time, these are humans, our family, our homes etc. In the case of David, he referred to God as his fortress; his help in times of need and tribulations. However, in the old testament, fortress was mostly a place, like a walled city supplied with food, ammunition etc where people could be defended in times of war.