Level designs should be designed inline with the players skill. The 1st level should therefore only include 1 Invader type and 1 path for the player to defend.
Level One: 1 Invader type. 1 path. 2 tower types.
Level Two: 1 Invader type. 2 paths. 2 tower types.
Level Three: 2 Invader types. 2 paths. 3 tower types.