Level progression advances a game through stages by increasing challenge and showing the player that progress has occurred. In a shoot-'em-up, one practical rule is to count enemies spawned during the current level, whether they are destroyed or not. section 13.4 section 1.5.3
A visible level transition
Start at level 1, display the level briefly, and count spawned enemies. When the count reaches 40, increment the level, reset the count, display the new level, and make the message disappear after its countdown expires.
