1. Introduction of the game:
Embark on a vibrant aerial adventure in "Bloons Supermonkey 2," where players dive into the sky as the heroic Super Monkey, tasked with defeating hordes of colorful balloons (bloons) that have invaded the Monkey Town. This action-packed, vertical scrolling shooter game combines fast-paced gameplay with strategy elements, requiring players to pop bloons, collect power-ups, and save the day.

2. All characters and levels of the game:
The game stars the Super Monkey, the player-controlled character, whose mission is to pop bloons using a range of weapons and abilities. As players progress, they encounter different Super Monkey avatars, each with unique skills. The game consists of several vibrant levels across multiple stages such as Monkey Lane, Bloon Dunes, and Frozen Fields, each presenting its own set of challenges and bloon varieties.

3. What attracts users most to the game:
Users are most fascinated by the game's satisfying balloon-popping mechanics, the wide array of weapon upgrades, and the colorful, eye-catching graphics. The satisfaction of clearing a sky filled with bloons and the strategic depth of optimizing the Super Monkey's upgrades to handle tougher challenges keeps players engaged.

4. Why it is popular:
"Bloons Supermonkey 2" is popular because it strikes a perfect balance between approachable gameplay and escalating difficulty, appealing to both casual and hardcore gamers. The game's intuitive controls, coupled with its addictive upgrade system and the gratification of achieving high scores, have contributed to its widespread popularity.

5. When the game was released and how many versions are currently available:
The game was initially released on mobile platforms in November 2016. As for versions, there's the original mobile release and periodic updates that have been provided by the developer to improve the game as well as to add new content, but it is still maintained as the single title of "Bloons Supermonkey 2."

6. The most difficult part of the game:
Many players find the later stages to be increasingly difficult, especially levels with MOAB-class bloons that require specific strategies and upgraded weapons to defeat. The high-density bloon waves and the introduction of different bloon types can provide quite the challenge.

7. Strengths and weaknesses of the game:
One of the strengths of "Bloons Supermonkey 2" lies in its addictive gameplay loop and rewarding progression system. The game's weakness, however, may stem from its repetitive nature, as the core gameplay involves consistently popping bloons across similar-looking levels. Accessibility to all upgrades without in-app purchases can also be a grind for some players.

8. What other games have been developed by the same developer:
"Bloons Supermonkey 2" was developed by Ninja Kiwi, a game development company well-known for its Bloons franchise. Other popular titles developed by Ninja Kiwi include "Bloons TD Battles," "Bloons TD 5," "Bloons TD 6," and "SAS: Zombie Assault 4," among others. Each game showcases Ninja Kiwi's strong ability to blend strategy, action, and tower defense elements to create engaging gaming experiences.