I did wonder if I was nearing the end of the game when the “level select” markers seemed to fill the entire map. Some of the final few levels are bloody hard though, or at least would be, if I hadn’t have played Original Mode for a fair while and unlocked some useful power-ups.
Level 19 was the hardest by far. It looks like a “normal” Invaders screen, but the invaders move up rather than down – trying to escape. That isn’t a problem to begin with, but once you only have one Invader left, it moves further away very quickly and is much harder to hit than when it would normally move towards you. Luckily, a combination of Rapid Fire and Support (two extra drone ships that shoot at the same time as you) both make it easier to shoot down.
Finally, the UFO Mothership, who wasn’t especially difficult, just a bit time consuming to destroy. And then it was dead, and the (shootable) credits rolled. End!