The Hell in a Cell Structure fails to keep anybody out during the Universal Championship Match
Braun Strowman wanted to fight Roman Reigns inside the Hell in a Cell structure because he thought that it would prevent Dean Ambrose and Seth Rollins from interfering.
Braun Strowman is an idiot.
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Or, he doesn't have the WWE Network
Because otherwise, he would have known that structures and match-stipulations that are meant to prevent outside interference, inevitably involve outside interference.
I do not think that Dean Ambrose and Seth Rollins will be the first to interfere in this match. Remember, they never interfered during Roman Reigns' Universal Championship defense against Finn Balor, even when it looked like Balor might win. That is not how the SHIELD rolls. They let each other fight their own battles. They only interfered when Braun Strowman tried to take advantage of a weakened Roman Reigns.
But Braun Strowman has found himself some friends; Drew McIntyre and Dolph Ziggler. And they will certainly attempt to interfere in his match, which will lead to Dean Ambrose and Seth Rollins showing up too.
The question is not whether there will be interference. It is who will inevitably knock out Mick Foley whilst running interference.