1. Introduction of the game:
Fate/Grand Order is a free-to-play, turn-based role-playing game developed by Delightworks using Unity, and published by Aniplex, a subsidiary of Sony Music Entertainment Japan. The game is based on the expansive Fate universe and draws upon history and mythology to create a rich, narrative-driven experience. Players take on the role of "Masters" and, with the help of "Servants," powerful heroes and spirits of myth and history, embark on a journey to prevent the catastrophic destruction of humanity.

2. All characters and levels of the game:
Fate/Grand Order boasts a diverse cast of characters, known as Servants, which are classified into several classes such as Saber, Archer, Lancer, Rider, Caster, Assassin, Berserker, Ruler, Avenger, Alter Ego, Moon Cancer, and Foreigner, each with its own strengths and weaknesses. The game's levels, known as "quests," are organized into a series of narrative-driven Story chapters and side missions, which are periodically updated and expanded through game updates and special events.

3. What attracts users most to the game:
Players are often drawn to Fate/Grand Order by its deep storytelling, character collection mechanics, strategic gameplay, and the rich universe it builds upon. The game's narrative is a significant draw, offering a multilayered story with twists and turns that resonate with fans of the original anime and newcomers alike. The gameplay requires forming teams of Servants to combat enemies, which appeals to players who enjoy strategy and team-building.

4. Why it is popular:
Fate/Grand Order's popularity can be attributed to its engaging and complex plotline, high-quality visuals, and compelling character designs. The roster of Servants from various historical, mythological, and literary backgrounds allows for a diverse and intriguing gameplay experience. The game also benefits from its association with the well-established Fate franchise, which has a dedicated fanbase. Regular content updates, events, and collaborations also play a significant role in maintaining the game's popularity.

5. When the game was released and how many versions are currently available:
Fate/Grand Order was initially released in Japan on July 30, 2015, followed by subsequent releases in China, Taiwan, Hong Kong, Macau, the United States, and Canada, among others. The global reach of the game means there are several localized versions available, tailored to their respective markets. These versions usually follow the Japanese release's lead, with some time lag for localizing content.

6. The most difficult part of the game:
One of the most challenging aspects of the game is the high difficulty of certain event quests and story chapters, which can require high-level Servants, strategic planning, and sometimes simply luck to complete. The game's gacha system can also be a challenge for players who struggle to obtain certain powerful or limited-edition Servants.

7. Strengths and weaknesses of the game:
The game's strengths lie in its compelling storyline, immense character depth, and the strategic element that tailors to different playstyles. The animation and artwork are other notable strengths. However, the game's weaknesses include a heavy reliance on the gacha system which can frustrate players due to its luck-based nature, and the potential for grinding to level up Servants and obtain essential items, which can be time-consuming.

8. What other games have been developed by the same developer:
Delightworks has also developed other games, although none have reached the same level of international acclaim as Fate/Grand Order. They have participated in projects such as Sakura Kakumei: Hanasaku Otome-tachi and are involved in the wider business of providing application development and operational services. However, Fate/Grand Order remains their flagship title and mainstay.