The difficulty ramps up through spawn numbers rather than enemy toughness - every enemy spawns every so slightly sooner every time - sounds like you got a really good run if you didnt notice it, cause it for sure ramps up! Theres no ending, its very much a "go till you die or break the game" at the moment.
Glad you enjoyed it! I was really pleased with the upgrade system and getting the weapons, they all feel and work differently!