I visited a couple parts of the fortification system on the Slovakian side in Komarno with my Slovakian friend back in August. The smaller fortresses and the walls sure are interesting, but the real gem is the main fortress, Pevnost in the centre of Komarno. For only 3 euros (student fare, it's either 4 or 5 for adults) you can get a guided tour of the fortress, lasting around 1 hour and a half and it is trully worth it. You get to see once the bigest army barracks in Europe, the Leopold's gate and the old fortress, magnificent views of the Danube and you even get to see the abandoned fortress on the inside on many occasions, including interesting little easter eggs like the swimming pool the Soviets built smack in the middle of a fort from the 12th century or the stables, where horses had to be lifted through the ceiling, since the doors were to narrow for them. The tour concludes with an interesting labyrint inside the old part of the fortress, that you get to experience in complete darkness, which is quite the experience.

There are some signs of parts of the fortress being renovated, but only in the newer part near the entrance and unfortunately only on a small scale. Renovating the fortress as a whole would be a huge overtaking and I doubt the economic feasibility of the project, since the complex trully is colosal and doing it just for touristic purposes would probably bring the city of Komarno on the brink of bankruptcy. Some of the smaller fortresses are in use though, the northernmost housing a museum, a roman lapidarium and, interestingly enough, a night club... 

Overall, it is a great opportunity to legally satisfy the urban explorer in you with many awesome photo opportunities in this collage of different historical eras and nations that have occupied it in the past.