The opening is exactly what you expect. Ships in rows, a gun at the bottom, shots going up. Then the wave ends and the game does something the arcade never did: the invaders that lived long enough to matter pass their traits on, and the next formation is built from them.
So the difficulty is not a number that goes up, it is your own play coming back at you. Sit in one corner and the descendants learn to come down that corner. Favour a firing pattern and the things that survive it are the ones it does not cover. It is a small idea executed cleanly, and it makes the fifth wave feel personal in a way the first one did not.