I played the demo and was a bit worried it would have a set route through the stages. Thankfully, after the first stage, you can pick between almost all of them. Unfortunately, most of the weapons seemed like shit to me, so it wasn't really interesting getting them.

Hard mode, unlocked after beating the game once, adds a lot more enemies per wave and remixes them up, including enemies from other stages. Maybe if I want to replay it in the future...