I had never played
Defender (or any of its variants) until jumping into
Aqua Kitty - Milk Mine Defender a few days ago. So far I'm having a lot of fun with it. I like the detailed 16-bit pixel art aesthetic, the soundtrack is fun, and it's got the "one more try" addictive arcade action in spades.
The gameplay basically involves darting back and forth across a short-ish 2D landscape, dodging bullets, shooting enemies, and protecting milk-mining kitties from abduction by certain aquatic invaders. The enemy types get pretty varied and there are the usual shmup-type powerups. There are both Easy and Normal difficulty levels, I think the only difference is the number of hits you can take when each stage begins, though. For hardcore challenge, you can enable "Arcade" mode, which allows you to collect gems to upgrade your ship (reminiscent of the Gradius upgrade system), but a single death is an instant game over.
If I have any criticisms of the game, one major annoyance for me is that unlocking the Normal-mode achievements don't also unlock the corresponding Easy ones. It's a good thing there's a certain amount of replay value, I guess. I could easily see myself going through all three game modes before getting tired of Aqua Kitty. If you like arcade-style shooting games, give it a try -- it's good fun.