They never update it, though. Which is fun for a bit, but then you kind of wish there was new stuff added when you start averaging 45 rounds per attempt. It gets so repetitive that, by round 60, you manufacture ways to make it more difficult ("Oh crap! Burnie no longer has ammo for his pistol which is on the other side!!"). I understand from a dev standpoint that it would be difficult to balance the effectiveness of certain characters with certain weapons. But Burnie and Gavin with the super low cost pistol (especially Burnie) in the higher rounds are way OP. And by that point, you likely have parts up the wazoo to make traps for the annoying armored alien.
Its a fun game, but seeing more content (I.e. Characters, weapons, or maps) would add a lot to the game. Even if it came in very small increments.
Xavier Sharp about Rooster Teeth vs. Zombiens