Early slot machines (such as Space Ace) had a very theme centric control design. User controls were built more by how they fit visually as they were by where they enabled the user.
Finding the Solution
Observing users showed a common “Bet Max -> Spin -> Double” action sequence. I divided the interface into four clusters; messaging, bet controls, play actions, and results.
(Petshop Payout reels by Jay Zak)