1. Introduction of the game
Plants vs. Zombies is an iconic tower defense strategy game where players must defend their garden from an onslaught of humorously zombified invaders. Developed by PopCap Games, this charming and light-hearted title combines strategy, humor, and a touch of horror in a unique blend that's approachable for players of all ages. The objective is simple yet enthralling: protect your home from zombies by strategically placing a variety of plants with different defensive and offensive abilities.

2. All characters and levels of the game
The game hosts an array of characters, primarily divided between the plants and the zombies. On the plants' side, you have the Sunshine-producing Sunflowers, the Peashooters, Cherry Bombs, Wall-nuts, and many more, each with unique abilities and upgrades. The zombies' ranks include the regular Zombie, Conehead Zombie, Dancing Zombie, and even a Gargantuar, all sporting different attributes and challenges. The game progresses through various levels across several modes, from the daytime backyard setting to nighttime and fog-filled levels, and even pool and roof levels, each introducing new challenges and requiring different strategies.

3. What attracts users most to the game
Users are most attracted to the game's charming and colorful art style, whimsical characters, the strategic depth masked by the game's simplicity, and the comedic elements that keep the mood light despite the zombie apocalypse. The rewarding progression system and the satisfaction of finding the perfect strategy to defeat waves of zombies also contribute to its broad appeal.

4. Why it is popular
Plants vs. Zombies gained popularity due to its easy-to-learn mechanics, humorous take on the zombie genre, and gradual difficulty scaling which hooks casual gamers and challenges strategic thinkers alike. Its unique theme of gardening meets the undead resonates with a wide audience, making it a staple among casual games.

5. When the game was released and how many versions are currently available
Plants vs. Zombies was initially released on May 5, 2009. Since then, it has expanded across multiple platforms and spawned a series of sequels and spin-offs, including 'Plants vs. Zombies 2: It’s About Time,' 'Plants vs. Zombies: Garden Warfare' series, and 'Plants vs. Zombies: Battle for Neighborville.' These titles have introduced new gameplay mechanics, characters, and multiplayer modes.

6. The most difficult part of the game
For many players, the most challenging parts of Plants vs. Zombies are the later levels, where the variety and density of zombies increase significantly. The game requires players to adapt their strategies and quickly manage resources like sunlight and plant cooldowns, especially during "Survival" mode and "Endless" levels where the difficulty continuously ramps up.

7. Strengths and weaknesses of the game
The strengths of the game lie in its accessibility, fun factor, and depth of content, which keeps players engaged for hours. The weakness might be its simplicity, which could be a turn-off for players seeking high-stakes or more complex gameplay. Furthermore, the pacing might feel slow for some, and the learning curve could become steep as players reach advanced stages.

8. What other games have been developed by the same developer
PopCap Games, the developer behind Plants vs. Zombies, is also famed for creating other beloved casual games such as Bejeweled, Peggle, Zuma, Bookworm, and many more. These games, like Plants vs. Zombies, are known for their simple yet addictive gameplay and have been enjoyed by millions of players worldwide.