The GG improvement is what you really want to avoid, as it usually can only save you from losing right now in an emergency situation - if you are on the winning side that Superfortification will only be at your front/in the fight for maybe 5 turns or even less. Meh.
I would have agreed on this one a few days ago but after a game I played yesterday, I think otherwise.
It was an OCC game as Babylon, emperor difficulty. As in the most of my OCC games I wanted to go for a cultural victory and I chose Bablyon due to their UA, which would help me in scientifical progress.
The ancient, classical and medieval eras went well. I spammed the necessary wonders (The great library, stonehenge, etc.) though I missed oracle. I was the first one to reach reneissance era and I managed to get some wonders there too. But then a great "misfortune" happened: Spain (which was the leading civilization militarywise) declared war on me. They had discovered the technologies that are required for cannons and came on my two longswordmen with a huge army of conqistadors. I hadn't expected this at all since they were far, far away on the map so I hadn't really built any army. After all the conqistadors finished off my longswordmen in two turns though they were seriously injured in the process and attacked on my city. They got my city over half way down before I could finish them off.
After this I didn't see any Spanish units in roughly twenty turns and Isabella even proposed a peace treaty with horrible conditions. I decided to build/buy a cannon and three longswordmen and the brandenburg gate as well to have the GG to build the citadel which I hadn't built ever before. I built the great fortress on a hill to the same side of my territory where the Spanish troops had been seen for the first time.
This time Isabella's army had no chance. A longswordman stationed in the citadel was able to hold off their whole army because the citadel bombarded the Spanish troops 3 damage per turn. In the end Isabella withdrew her army and sued for peace again with the same conditions.
Basically the situation didn't change at all during the rest of the game except that at one point Wu Zetian also DOWed me. They were quite successful as they used a different route to attack my city than Isabella, but they didn't do more than about 3 damage on my city. I used the next GG to build a citadel on that direction and as my longswordmen were upgrade to riflemen and my cannon was upgraded to an artillery, I was able to hold both the Spanish and the Chinese troops off. In the end Isabella DOWed on Wu and took her capital (which was the only Chinese city in the game) in just a couple of turns.
The Spanish kept fighting just as badly as before even though they kept constantly sending their troops on my territory. After their first attack they never managed to touch my city at all due to the two citadels. Of course the setting was ideal since the other side of my territory was protected by mountains so the Spanish had no other routes but the two with the citadels blocking their way. Isabella and Kamehameha managed to catch me up in techs and my war with Isabella became modern. I was lucky enough to have oil to protect my territory with 2-4 fighters but Isabella built even more of them and almost finished my mech infantries in the citadels before the planes were finally destroyed.
After all I actually won the game despite of having 200 turns of war with Isabella (+ Wu for ~50 turns). Kamehameha was a few turns away from winning scientifical victory though. Spain also finished the manhattan project about 30 turns before my victory but luckily I never saw any nukes. In the end I doubt that I could have defended my territory for more than about 10-30 turns after the first Spanish invasion had I not build the citadels. The game was one of the most interesting that I ever played in Civ V and it changed my opinion of citadels. Another funny fact was that Isabella proposed to make a peace treaty ~20 times in the game never changing her conditions despite of losing dozens of units.
Now of course citadels are mainly useful for very small empires that can't field a big army. Also I doubt that they would be effective in multiplayer games since as a human player I could have found a strategy to pass the fortresses without getting all my units killed.
I know that it might be a bit tl;dr, but if you managed to read it, I applaud you.
Edit: Added a screenshot. It's 7 turns before the end of the game as you can see. Soon after this Isabella launched her final attack which consisted of modern armors, infantries and artilleries but even with those she didn't get past the citadels without having her units down to about 1-3 health points. Of course she could have used the northern route but.. She just didn't. We should praise the AI tactics x)