Tier 1 presents the most Invasion unique game play of all, because it lacks power weapons, relying instead upon the Invader Middle Team outwitting the Defender Middle Team in a Dual Territory Tier. Straight out of the spawn, the Middle Team is fully equipped to operate in their role as the wild card team to help either the Alpha Team or Bravo Team.
To make this type of Game Play possible, the paths that are offered to the three Invader Teams must permit lateral movement between the left and right sides of the map, as they move from their spawns toward the Objectives. Of course, these lateral paths are generally for the Middle Team to use, since the Alpha and Bravo Teams are used to draw the Defender Alpha and Bravo Teams away from each other.
A good Invasion map will require that the Invaders move off their spawns immediately to take cover to add story of an initial landing of invasion forces. An urgency is created by exposing their spawn, creating a sort of duck shoot for the Defenders. Like the landing at Normandy, the Invaders' initial push is fraught with a lack of cover. And just as taking control of the beach at Normandy was critical for that campaign to establish a foot hold to move forward from, Invasion makes taking Tier 1 necessary for the exact same reason.
From spawn, the Invaders must immediately see where they should go to achieve cover. This cover should then allow them to make decisions on where they want to move next, especially in the case of the Middle Team. At each point of cover, the Invader should be presented with two paths to choose from. More importantly, from the Invader's perspective, each path should be encouraging by offering additional cover. If one direction offers cover and the other does not, then from the perspective of the Invader, there is little practical choice to be made.
Too many paths make the game play boring, as the opposing players chase each other around a countless number of obstacles rather than engage each other for control of specific areas that offer positions of power. Too few paths make the game play boring, as the Invaders' paths are too predictable.
The paths that lead to the Objectives should be engaging to make fire fights fun. Defenders should have ways to keep their opponent pinned down until one side emerges the victor and begins to control that area. This is more of a generic level design concept, but is even more applicable to Invasion, because in Invasion there is a forward progress that the Invaders must make with very specific direction.
Summary
Tier 1 requires plenty of lateral movement for the Invader Middle Team to change their movements and add their numbers to either the Alpha Team or the Bravo Team in dynamic game play.
A good Invasion map will give urgency to the Invaders to move off spawn and take cover, adding to the Story of Tier 1 being a landing of invasion forces.
Paths should be encouraging and engaging, allowing Defenders to pin down Invaders until the Invaders adapt, improvise, and overcome.