Both Spire and Boneyard use electronic barriers to prevent players from entering the Tier 2 areas. Breakpoint has a different type of barrier, obviously for variety sake; but the key point to catch here is that it has a barrier - because a barrier is a fundamental Game Element of Invasion.
Let's take this a step further. The barrier is more than the electronic fence, retracting bridge, locked gates, or tunnel door. The barrier is found in the very shape of the landscape of the map itself. The barrier is a foundational Map Element - that is, a foundation pillar of the Map.
Breakpoint has its backbone and gorge as natural barriers with a bridge and tunnels as access routes through those barriers. Those access routes are closed off, in effect. But the backbone and the gorge remain the natural barrier that the map presents to the players. The barrier wasn't added to Breakpoint. The barrier is Breakpoint.
Because the barrier is literally part of the foundation of the Canvas, it builds cohesion between the play areas of Tier 1 and Tier 2. That is, players don't see the two halves as two maps, but rather as one whole map. At the same time, the barrier provides the natural means to isolate Tier 1 during Tier 1. The barrier contributes to the theme, the character, the feel of playing on the Canvas, and makes the barrier's presence a natural experience through out the Game.
Boneyard's barrier is not the electronic fence, but the entire ship in the middle of the canvas. The electronic fence merely plugs the holes and makes the ship a functional barrier. The ship itself gives the map a character, it brings the map alive with a realistic obstacle to overcome.
Spire's barrier is the crest at the top of the rise, before you descend down into the bowl where the Spire sits. The electronic fence simply plugs the holes and makes the natural surrounding an effective barrier. The rise, the crest, the bowl, and the Spire all contribute the feel of being on the Spire map. Again, the barrier is integral to the canvas, not an after thought forged into the map.
Notice also that the barrier is intended to prevent the Invaders from advancing to the Defender strong hold. Though Boneyard and Breakpoint each isolate both Species to Tier 1 during Tier 1, Spire isolates only the Invaders. Again, this is fully compatible with the Story and in no way interferes with Game Play.
And the presence of a barrier gives the Tier 1 Objectives a purpose. The Story comes alive when the Tier 1 Objective is captured and the barrier is breached. The Game Play no longer takes on capturing a Game Piece on a Game Board, but rather penetrating the barrier to push forward into the enemy strong hold and onto the ultimate Objective.
Barriers add cohesion between Tier 1 and Tier 2, can help immerse the players into a theme, a place, a campaign.
Barriers are fundamental Game Elements and foundational Map Elements that bind the two halves into one map.
Barriers provide purpose for Tier 1 Objective and help make the Story and Game Play come alive.