The final world in the original Plants vs. Zombies is The Roof.
Adventure Mode is divided into five stages of ten levels each: Day, Night, Pool, Fog, and Roof. The Roof is the climactic setting, where zombies attack across sloped rooftop lanes rather than a flat lawn. That change affects the strategy because ordinary straight-firing plants cannot reliably shoot uphill; players must use catapult plants such as Cabbage-pult, Kernel-pult, and Melon-pult.
The Roof is also where the game’s final Adventure Mode encounter takes place. At level 5-10, the player faces Dr. Zomboss, the zombie leader, in a large machine that launches vehicles and other hazards onto the roof. The encounter is the conclusion of the original campaign, although completing it unlocks additional activities, including Mini-Games, Puzzle, and Survival modes.
A common mix-up is treating Pool or Fog as the ending stage because both introduce major mechanics and memorable zombie types. They are earlier environments. The Roof is the fifth and final Adventure Mode stage, and its sloped battlefield provides the game’s last major tactical twist.