When we stripped the game mechanics back to basics we decided to give the player a regular projectile to fire. This avoided us having to design screens with large areas of vertical space ( to all throwing of enemies such as in Stackies or to jump on enemies heads ). Pressing fire would spawn a projectile which would travel in the same direction as the player. For simplicity ( and a bit of a challenge ) we limited the player to only having one of these projectiles on screen at a time.
We went with the idea of a player having to collect all the pick-ups on a level in order to progress to the next one. We'd had some code for pick-up collection left over from the adventure game and it seemed a good fit. Having to collect all the pick-ups forces the player to fully explore the screen and plan a route past all the obstacles. We wanted to maximise the player's time per screen as we were keeping with only having one screen per level.
When the projectile went in we first had it so that enemies were killed on collision with it. However removing enemies from the screen made each screen get progressively easier! As you shot at enemies and removed them from the map, the number of obstacles the player faced would also reduce. We debated on some alternatives, in particular the idea of having enemy spawners in the level. This would have spawned in a replacement enemy whenever one was killed. But this also had knock on effects to the screen design :-( More screen space would have to go to these spawners, which may not be active for much of a level's duration. The additional problem of getting the new enemies into a position where they blocked the player's path also proved to be tricky.
However the surprisingly simple and elegant idea of stunning enemies, rather than killing them came up as a way around the problem and provided a good challenge for the player. This meant that enemies would only be temporarily out of the picture, long enough for the player to get by, before they'd wake up and continue on their way.
The screen above used to be the first level in the game and was used when testing out all the above mechanics. It later became screen 7 in the final game.