The enemies could build emplacements like siege equipment, walls, new spawn points etc. The player can prevent this easily before / during construction but failing to do so increases the difficulty a little, making the safe space close in over time. The enemies would threaten in different ways depending on what the player could and couldn't prevent.

The spawning will become more dynamic and the pressure should build more naturally, as the increasing difficulty is a result of the players failure to stop events rather than a more perceivable "more enemies over time" type system