It sounds pretty useless, but it absolutely saved me in a game, and not even in a big choke point (details below). I was playing an Earth game and started in the US as the only player in the Americas. When I finally started invading other civs via the Alaska/Russia "bridge", I had some success until I went for the English Capital city. I realized I had been too aggressive and was in danger of losing all of my cities in Asia. I was also greatly in debt so using a GG was a good option (and I was down to only a 3 turn golden age for using great people). Therefore, I used it to create a citadel between the capital and my closest city and was (barely) able to survive thanks in large part to it. Once I turned the economy around and could afford an army again, I steamrolled the rest of the civs and won easily. Now, this was only my 2nd ciV game so I made plenty of mistakes that got me into my situation, but once I was there the citadel helped me survive until I recovered.