Again I've been looking into too many other game projects. A few hours have been spent on this one though.
Previously purely random velocities - with an upper limit. Occasionally an alien would move very slow or have no velocity in either the x- or y-direction. Game difficulty progress relied solely on the increasing number of aliens per level. Now there's upper and lower limits for each level.
The first level almost appear a bit boring after some play-testing, - if I get lazy they still get me. With the new velocities, I'm having a rather hard time getting through to the end. That's probably how it should be.
I've had a long standing bug at the beginning of level 7. Obviously some memory areas overlapping. Finally went through the Scratch-Pad layout and found it. Hooray.
The circle based system seems to work fine, but I'm now going to implement a rectangle system instead. Hopefully to save a relatively large amount of CPU. At any given time there's between 1 and 22 possible collisions to look out for.
All for now.