1. **Introduction of the Game:**
"Sword Art Online: Integral Factor" is a thrilling mobile MMORPG set in the immersive universe of the "Sword Art Online" anime series. Developed by Bandai Namco Entertainment, this game gives players the unique opportunity to become the protagonist, striving to escape from the virtual reality world of Aincrad. With a blend of adventure, role-playing, and collaboration with other players, the game reimagines the series' plotline, placing players' very survival at stake, as they fight against monsters and level bosses to reach the 100th floor and break free from the game.

2. **All Characters and Levels of the Game:**
Players take on the role of their customized avatar, who is among the many trapped within the game. They can team up with familiar characters from the original series, such as Kirito, Asuna, and many others. Each character brings their unique abilities and skills to the party. As for levels, "Integral Factor" challenges players to clear floor after floor within Aincrad, with each level possessing more challenging foes and intricate maze-like designs. The narrative unfolds as players ascend through these levels.

3. **What Attracts Users Most to the Game:**
One of the most attractive aspects of "Sword Art Online: Integral Factor" is its faithful recreation of the Aincrad setting, which allows fans of the series to live out their own SAO saga. The game's MMORPG elements encourage cooperative play, which is a significant draw. The skill-based combat system, level progression, and the feature to craft and upgrade equipment offer an engaging loop that keeps the players invested. Additionally, the game's events and updates that tie-in with the anime increase its appeal.

4. **Why It Is Popular:**
The game's popularity can largely be attributed to its connection to the "Sword Art Online" franchise. It offers fans an interactive way to experience the story and a virtual reality world, which many have only watched or read about before. The game's MMORPG mechanics allow for a sense of growth and development, and the social aspect of teaming up with others to tackle challenges together mirrors the themes of the anime and adds to its popularity.

5. **When the Game Was Released and How Many Versions Are Currently Available:**
"Sword Art Online: Integral Factor" was released in 2018. Since its initial launch, the game has undergone various updates and patches but remains a singular version that continues to evolve with new content and features to enhance player experience. It’s important to check the respective app stores or the game's official site for the latest information.

6. **The Most Difficult Part of the Game:**
One of the most challenging aspects of the game for many players is the high-level floor bosses. These bosses require significant preparation, strategy, and coordination when faced in teams. Additionally, mastering the game's skill system and effectively managing and upgrading equipment can be a daunting task that requires a significant investment of time and resources.

7. **Strengths and Weaknesses of the Game:**
Strengths of "Sword Art Online: Integral Factor" include its strong storyline, which is engaging for fans and newcomers alike, as well as its engaging combat and team-play mechanics. However, weaknesses might include a potential grind to progress at higher levels, which may deter casual gamers, and the reliance on in-app purchases for faster progression, which can sometimes lead to a pay-to-win perception.

8. **What Other Games Have Been Developed by the Same Developer:**
Bandai Namco Entertainment has developed numerous other games, spanning various genres and IPs. These include "Dragon Ball Z Dokkan Battle," "Naruto X Boruto: Ninja Voltage," "One Piece Treasure Cruise," and other "Sword Art Online" titles such as "Memory Defrag" and "Alicization Rising Steel."