Lobby, intermission, then waves until you go down. Zombie Rush is the arcade shape of that idea with one change that matters: coins are banked on the kill, never on survival. Getting wiped on wave nineteen still pays for the upgrade that gets you to twenty-one. Nothing is ever a wasted run.
Three arenas ring a central lobby, each running its own match loop. You join one by walking onto its platform — everyone standing there goes in together, stepping off takes you out, and nothing queues you automatically. Harder arenas are the same wave curve started further along, paying a higher multiplier for the privilege.
The progression is the load-bearing part, and it was tuned by simulation rather than by feel: a hundred solo runs against the real config, with tests that fail if the curve either stalls out or snowballs. Plateaus sit deliberately on the boss waves.