I'll get this out of the way first: the budget of this film isn't fantastic. Nearly all of the action takes place in the titular Fortress, which consists of a handful of sets used two or three times over. There is also a truly awful make up job - a 'tattoo' that more resembles an enthusiastic black marker.
That aside...
Despite said bad make up, Fortress features some extremely good practical effects. While avoiding spoilers, some very messy deaths are involved with characters being literally blown apart. These look frankly fantastic, especially (as I'm sure you've heard from every other critic ever) compared to CGI deaths in PG-13 films today. There's something visceral and authentic about them...the idea that what you're seeing is really happening.
And despite being restricted to one location, Fortress certainly makes the most of what it has. The Fortress is very well designed, packed with containment systems that the protagonist turns against it. Granted, there is some silliness involved - apparently, there is only one non-lethal way to subdue prisoners, and after that it's straight on to 'blow-to-bits'. But -and this is important- all of it is for a reason. You don't get the sense that something is there just to facilitate someone using it to escape, and indeed the design appears to be based on a real-life idea from Britain - the Panopticon, emphasising prisoners' fear of being watched more than actual effort spent watching them.
Characters, then. Honestly, Christopher Lambert's acting is the worst part of the film. In fact, I get the impression that a certain sequence was made just for him, since all he needs to do is zone out for a few hours. It's as if his batteries ran out one day, but the crew had to keep filming so they quickly wrote in an in-story excuse. His character isn't that interesting either - ex-army, highly decorated, a typical 90s interchangeable backstory with little impact on the plot. However, this is more than made up for by the antagonist, Prison Director Poe.
Unlike normal villains of the genre, Poe isn't a cartoon character given form - indeed, Kurtwood Smith is rather subdued even by normal villain standards, contrasting with his earlier role as Clarence Boddicker. And also unlike Boddicker, Poe's actually a fairly interesting character - though an aloof, even sadistic Warden, we do see what little personal life he has, and are continually reminded in his scenes that he's still a person - he has tastes, even loves, and by the end of the film it's not even clear whether he's the real bad guy. The rest of the supporting cast do an okay job - I can't think of a stand out performance though everyone does their jobs well. Honourable mention goes to the writers and the director for giving a female character many good action scenes and half of a subplot, even when it would have been incredibly easy to neglect her as post-victory booty.
The setting, I should mention, has a very nice atmosphere. Too many films forget that these days, instead going for a more realistic, bland appearance - comparing the first and second of Nolan's Batman trilogy shows it blatantly. Fortress has a dark, smoky look, which helps greatly both to maintain an oppressive, cyberpunk feel, and hide bad FX which is a nice bonus.
This isn't a 'bad but fun film' in my opinion, though some may see it that way. It has that look, sure, and if the creators had cast Arnie in the role then it well may have turned out like that. But it has some depth - beyond the exploding bodies and in places absent acting there is thought. If anything, I'd liken it to a Paul Verhoeven film - entertaining, but still immersive, maybe even though provoking.
