floating fortress

The floating fortress attacked the Cumberland, ramming that vessel, and breaking a great hole in its side.

The Queen Elizabeth was there, looking for all the world like a floating fortress.

Looked at in another way, we may say that a battleship is a floating fortress.

I was quite bewildered as I ran through the passages and endless nooks of this floating fortress.

All the men swung their hammocks on board their floating fortress, and were quite secure from any intrusion of the savages.