1. Introduction of the Game
Fortnite is a cultural phenomenon and a wildly popular online multiplayer game developed and published by Epic Games. Initially released in 2017, Fortnite is distinguished by its vibrant graphics, captivating gameplay modes, and the unique blend of building mechanics with intense battle royale action. Players are dropped onto an island and must scavenge for weapons and resources while battling to be the last person or team standing.

2. All Characters and Levels of the Game
Fortnite features a diverse range of characters known as "skins," which players can acquire and use to customize their appearance in the game. These skins come in various designs, often inspired by different themes, popular culture, or special collaborations. The levels in Fortnite are not traditional game levels; instead, the game has a continuously evolving landscape within its island that changes each season, which itself can be considered as different ‘levels’ of experience for players.

3. What Attracts Users Most to the Game
The game's attraction lies in its engaging and competitive gameplay, social interaction, regular updates, and the thrill of building and creating structures on-the-fly. Additionally, the ever-evolving narrative and special events, such as in-game concerts and movie tie-ins, keep the game fresh and exciting.

4. Why It Is Popular
Fortnite has gained popularity due to its free-to-play model, cross-platform playability, and vibrant community. Its accessibility on almost all gaming platforms allows for a broad audience to participate. Also, the game's constant evolution with new content, challenges, and its blend of gaming and popular culture keeps players hooked.

5. When the Game was Released and How Many Versions Are Currently Available
Fortnite was released on July 25, 2017. Its main game modes include Fortnite: Save the World, a cooperative shooter-survival game for up to four players; and Fortnite Battle Royale, a free-to-play battle royale game where up to 100 players fight to be the last person standing. While the core versions remain constant, the game has gone through numerous seasons, each effectively being a new version, with the state of the game at that time changing significantly through patches and updates.

6. The Most Difficult Part of the Game
One of the most challenging aspects of Fortnite is mastering the building mechanics, which are crucial for defense and gaining a tactical advantage over opponents. The steep learning curve of building effectively while engaging in combat can be quite difficult, especially for new players competing against experienced ones.

7. Strengths and Weaknesses of the Game
Strengths of Fortnite include its unique building mechanics, regular content updates, and strong community engagement. Weaknesses may include an occasionally steep learning curve for new players and a monetization model that can sometimes be perceived as aggressive towards the purchase of cosmetic items.

8. What Other Games Have Been Developed by the Same Developer
Epic Games, the developer behind Fortnite, is also known for creating the Unreal series, Gears of War series, and the Infinity Blade series for mobile platforms. They are also the creators of the Unreal Engine, a widely used game development engine in the industry.