Thanks very much for the balanced comments sometimes99er.
The unpredictability was intentional as I found it a lot of fun ... I have adjusted the difficulty for breaking the wall so it is easier at the start.
I actually experimented with making the river more predictable but the code wouldn't fit into the memory. I have kept the original lean looking code. I have slightly adjusted the routine so Bertie finds himself on dry land after jumping in the river.
Levels one to four are finished. I just need to think of something fun to put in level 5 and what to do after level 5 (just repeat the levels again maybe ...). Each level has a different alien (which I lifted from my invaders game). Also this game is intended as a follow on from after the invaders have landed so that's why they are included in this game.
Level 1: The city
Level 2: Countryside (as it is here in Finland or some end of the world winter wasteland)
Level 3: City again but now when the alien bumps into a wall they knock down a brick and it blocks the road
Level 4: The river turns to ice every now and again making it easy to cross
Level 5: City again but nothing special happening here yet
Level 6: A dark void. Perhaps representing the end of the world or outer space? This of course will not be in the final version of the game ... I'm just being a bit lazy to make a nice ending at the moment.
There's a few small bugs in the game that need to be ironed out. Next version will be the finished version.