There isn't any real progression, for the defense missions where I spent most of my time you choose a diificulty which gives you a set amount of funds and from that point you have everything in the game at your disposal and you get to call in what you can within your budget. As the mission goes on you can call in back up if you saved some funds.
There are no vehicles, although there are turrets and similar devices that are mechanical in nature that you can control.
Environments are destructible but keep in mind that everything is made out of the same material, I'm not really well versed in the engine but basically everything outside of the actual characters are made out of similar material. If you ever played Worms it's like that but a bit more involved
There are single player missions, with varied objectives although I think the defense stuff is the real meat. It's all one off stuff though, the campaign is available but listed as imcomplete. I haven't spent any time with it. There is multiplayer but only locally. No online or lan support, which makes sense given the amount of particles going around at any one time but it's still a shame.
As I said though, it's a super neat game with an incredible physics system that makes it a blast. I just haven't seen much progress in years.
