I wanted the game to have waves of enemies, where 4 or 5 of the same type will appear and move in a predictable pattern towards the player.  I placed some enemies in the stage and it was getting a bit tiresome moving them around if I wanted to move a wave to a different part of the stage.  So instead of placing every individual enemy, I created a 'lead' enemy of just 1 type.  All the enemies are flagged as inactive until they are drawn on screen.  As soon as this enemy is made active, it starts moving and animating.   I also added a spawn code to the lead enemy where it created multiples of itself and sets a speed and direction.  That way if I want to re-position an enemy on the stage, I only have to do it once and move the lead enemy.

I also started adding in a bit of collision detection, to test out the gameplay.  Each bullet fired by the enemy has a 'bounced' flag.  If it is bounced off the players shield, this flag is set.  This way, enemies don't blow each other up with their own bullets.  When a collision occurs, first I check to see if the bullet's bounced flag has been set.  If not do nothing, if yes then create an explosion and destroy the enemy.

I also have some ideas for power-ups, one will make the player move slightly faster, another makes the shield stay active for a longer time.  There's a red bar at the bottom of the screen which measures how long the shield will last.  When I get around to doing some sounds I can put in some cues which alert the player when it is low or about to run out.