Impeding your progress are any bits you accidentally leave unattached to the greater structure, which eventually turn red and start multiplying themselves to take over the empty spaces on the board -- but, they're easily dealt with by building boxes next to them, as they'll be destroyed when the boxes explode. You continue on building and blasting as the speed gets faster and faster, and each level flows seemlessly into the next until you either choose to quit or crash out from an overwhelming onslaught of the evil red bits.
I think puzzler fans will love this design -- it really is like a version of Tetris that was never made. It could've even been designed by the Tetris Company and released as "Tetris Bit Blaster" or something and no one would've known the difference. Because of that familiarity with the seven standard shapes, the control can take a bit to get used to -- you expect the pieces to rotate the same way, but the buttons aren't assigned exactly like Tetris here.