Thanks for the comments, people. I am trying to polish some details because, as the game itself is not as complex as others I've done, I'd like it to be full of little details.
I have been implementing and testing some power-ups for those classic shooter lovers

. For now these are the extras available and their graphic representations:
- Extra life (heart)
- Destroy all enemies (bomb)
- Quick shot (joystick, due to evident reasons)
- Super-charged shot that does not disappear when hitting enemies (skull)
Still testing them, but beware some may not survive in the final version. Probably the first two will and also probably the super-charged shot. The quick shot needs more testing because too fast shooting creates lots of lasers moving around overloading the graphic engine. If drawing everything takes too much, I start losing frames which is something I want to avoid.
I think I found a good balance between shot speed without losing performance, but need more testing.
I have another power up in mind involving a temporary freeze of the enemies (graphic: fruit), but that needs to be coded. And yet another idea with a very rare powerup that could make you advance the level without actually solving the switch puzzle. But not sure if this will see the light in the end.
There are another 8 different items, which are just for recharging your retr-o-meter bar, which (as I already said) will give you an extra life when full.
Oh, and I have added a first bonus screen 'survive the onslaught' (suggested by Dbug, as usual!) where you have to finish all the incoming enemies to advance to the next mother ship.
Still some work to do, but this is taking shape
Don't be shy and drop here your comments, thoughts, suggestions... Feedback is always welcome, believe me.