In hindsight, it feels obvious. The joy of single-player Descent was always about tumbling through space, trying to get a bead on the vicious robots trying to kill you while dodging laser fire and spoofing homing missiles. But the most tense moments--and often the most thrilling ones--were when you were down to a sliver of life, desperately trying to stay alive long enough to find a shield powerup and hoping the robots around the next corner wouldn't flay you alive. Imagine a Descent successor that distilled the single-player campaign down to the moments where your life hangs in the balance, and you have Sublevel Zero.
The biggest thing Sublevel Zero gets right is the feeling of movement. Like the official modernization of the Descent formula, Descent Underground, but unlike so many other six-degrees-of-freedom games, Sublevel Zero nails the sensation of zooming through claustrophobic mine tunnels in a zero-g fighter ship. The second biggest thing Sublevel Zero gets right, though, is in carving out its own unique identity. It's a roguelike, you see, so death is permanent. There's that desperation again, built right into the game design. Randomness is part of Sublevel Zero's DNA as well: all the levels are procedurally generated, and your arsenal of weapons is completely dependent on what you find scattered about and what you can craft. Even the nanocarts you earn at the end of a level are randomized, meaning you can never be quite sure what benefits will be open to you each time.
Because there's so much uncertainty to each run, and because you only have one life to live, improvisation is the name of the game. This helps solve one small issue with Descent's campaigns, which is that sometimes the variety of weapons feels superfluous. The bread and butter weapons like the plasma, quad lasers and vulcan/gauss were often all you needed, leaving the more exotic weapons to collect dust. In Sublevel Zero, sometimes you have no choice but to pick up that Firebolt or those grenades and figure out the best way to use them.
I've played a bunch of roguelikes in my time, but most of them never managed to click with me. Either they felt too much like the product of luck, or they required skills that I didn't really have and didn't feel like training for (hello, every platformer roguelike ever). Sublevel Zero is the first roguelike where I felt like I could reach the end even from the very beginning of the game, once I learned the patterns and remembered how to fly properly. It never once felt unfair, and even when I made dumb mistakes (ask me about my RAGEQUIT achievement) it was easy to start again and hope this run would be the one. More than any other game to date, Sublevel Zero is the inheritor to the Descent legacy. Sigtrap should be proud.