An arcade-style game. What that means is: tight length, tight design, and high difficulty.
Multiple difficulty modes, from Expert (the hardest mode, which the game will be initially designed around) down to Novice mode (which most people will be able to beat).
You have one life, and there are no levels per se. Instead, the game will be one unbroken "shot" from start to finish, beginning when the player is dropped off in the first level (or last checkpoint, on lower difficulties) and ending when he/she either dies or defeats the last boss. (Remember how Sonic 3 & Knuckles had Act 1 and Act 2 levels seamlessly connected? Now extend that to an entire game.)
"Flow" is extremely important to us in designing Steel Assault. Many modern 2D platformers take the route of "bite-sized" design: hundreds of levels or checkpoints, each less than a minute long, and with infinite lives. But while "bite-sized" design is now a popular paradigm for platformers, we believe that this design choice severely limits a game's flow and tension, and (even more importantly) player immersion in a game's atmosphere and world
Fundamentally, we believe that retro styles should be an invitation for developers to push the limitations, not an excuse to fall back on them. We don't have any nostalgic pretenses or axes to grind with modern videogames, and we're not trying to sell you some tired rhetoric about "the good old days". We just want to make a kickass 2D action game.