I'm working on a simple arcade style game similar to Robotron 2084 (http://en.wikipedia.org/wiki/Robotron_2084).  I wanted to know what is the best way to manage the wave of attackers for each stage?  I was thinking of breaking up each attack wave into a "timeline".  That way i can control the interval and frequency at which enemies are spawned into the game.  Another idea would be to place each "attack wave" in a queue and at each time step peek into the queue and check whatever or not it we should pop the queue (depending on whatever all enemies in the current wave have been destroyed or enough time has elapsed to start a new attack wave).  Any suggestions?