Some of them come down after you
Every so often one peels out of the formation and dives, aiming roughly where you are standing. It is faster than the shuffle and it does not care about the row it left, so the safe habit of parking under the fleet and firing upward stops being safe.
Shoot a diver and it is worth triple. Miss and you move.
The speed-up was a hardware fault
Fewer aliens on screen meant the original machine could redraw them faster, so the fleet accelerated on its own as you cleared it. Nobody planned that. It was kept because it turned out to be the entire difficulty curve, and every rebuild since copies a 1978 limitation deliberately.
The last two move at roughly three times the pace of a full formation, which is why so many waves end one alien short.
Bunkers wear out from both sides
They are made of blocks, not health bars, and your own shots take chunks out of them on the way up. Fire through your own cover often enough and you have carved a window for the bombs to come back through.
Arrows to move, space to fire, or the three buttons if you are on a touchscreen. If you want the other arcade rebuild, the wall at google block breaker is slower pressure, and google pac-man is the one that was never taken down.