It was nearly called Lawn of the Dead
George Fan built the game after enjoying the defend-the-path maps that players had been making inside other games, and his working title was a pun on a famous zombie film. Lawyers took a dim view, the name went, and PLANTS VS ZOMBIES arrived in 2009 instead.
His design goal was unusual for a strategy game: he wanted something his girlfriend, who did not play games at all, could sit down with and enjoy. That one constraint shaped everything about how the game introduces itself.
One idea at a time
There is no tutorial screen. The first stage hands you a single PEASHOOTER and a few ZOMBIES shuffling down one lane. The next adds the SUNFLOWER, and with it the notion that you must grow an income before you can afford defences. The one after adds a second lane.
Nothing is ever explained in words, and nothing new appears until the previous idea has been used successfully. By the time NIGHT falls, the POOL opens up and zombies start arriving wearing buckets, a player is juggling five or six concepts at once without having read a single instruction.
Sunshine is the actual game
Every decision comes down to SUN. The SUNFLOWER produces it slowly; everything else spends it. Plant your defences too early and you never build the income to survive the later waves. Build economy for too long and the ZOMBIES are at the HOUSE.
Invest or defend is the oldest question in strategy games, and here it is served on a suburban LAWN with a smiling plant and a cartoon sun. A SHOVEL lets you dig up what you got wrong, at the cost of everything you spent on it.
The plants sit at either end of that decision. A WALL NUT is cheap and does nothing whatsoever except stand in the way absorbing bites; a CHERRY BOMB costs a small fortune and clears an entire crowd exactly once. Almost the whole game lives in the space between those two.
The jokes are doing serious work
These are the least frightening undead in fiction. They shuffle, they mumble vaguely about BRAINS, and they defend themselves with a traffic CONE or an upturned BUCKET, on the reasonable grounds that this is what a suburban garden has to offer.
The comedy earns its place. A game about the dead besieging your home could be thoroughly unpleasant; making the attackers ridiculous means a seven-year-old can enjoy a horror-shaped game without a moment's worry. The song over the end credits, sung from the zombies' point of view, became popular entirely on its own.
The last line of defence
Each lane has a MOWER parked at the end of it. Let a zombie all the way through and it fires once, clears that lane completely, and is gone for the rest of the stage.
It is a lovely piece of design. A beginner survives their first serious mistake rather than starting again, while an experienced player treats those mowers as a resource to be guarded — because a lane that has already used its MOWER is the one lane you really cannot afford to lose. Everything in the GARDEN, in the end, is about what you can spare.