Exactly how will keys be used?
As I've said, on the data layer we could set a security level of a door, therefore you can pick up a security pass that will allow you through several doors, as we've got the vis in we need to split the level up a lot so having 1 key 1 door is just too clunky. Of course you'll still have red, green and blue keys etc for speicalised doors. When do we start writing the save/load functions.
I think if there is a save you should only be able to save inside the intex systems, maybe saving will cost you money to stop you saving too often?But I think you should only save at the end/beginning of each level. Of course this is the easiest solution as you don't need to store the positions of all the aliens, alien states, etc etc etc.
I think the next major jobs that need to be achieved are improving the collision detection with player/alien, bullet/alien, alien/player, alien/wall and player/wall.
Get the credits in (should take about 5 minutes to add a graphic and a type of credits) of course share credits needs to be taken into account with 2 player option.