1. Introduction of the game:
Sniper 3D Gun Shooter is an action-packed shooting game that challenges players to take on the role of a highly skilled sniper. Developed by Fun Games For Free, the game offers a combination of thrilling missions, precise shooting mechanics, and an immersive 3D environment that draws players into the intense world of a professional sniper.

2. All characters and levels of the game:
While Sniper 3D does not focus on character development as some narrative-driven games do, the player takes on the persona of an unnamed sniper. Throughout the game, the characters are mainly targets that the player must eliminate to progress through the levels. The game features a multitude of levels that vary in difficulty and setting, ranging from urban landscapes to more remote locations, ensuring a diverse gameplay experience that continues to challenge players as they advance.

3. What attracts users most to the game:
Players are most attracted to Sniper 3D for its realistic sniping mechanics, high-quality graphics, and the satisfaction of executing perfect shots. The game is also lauded for its varied missions, which can involve anything from rescuing hostages to taking down high-profile targets, providing a sense of purpose and urgency that keeps gamers engaged.

4. Why it is popular:
Sniper 3D has gained popularity due to its intuitive controls, engaging gameplay loop, and the ability for players to upgrade their sniper rifle and gear, which adds an RPG element to the shooting experience. The game's availability as a free-to-play title across multiple platforms, including mobile devices, has also contributed to its widespread appeal.

5. The most difficult part of the game:
The most challenging aspect of Sniper 3D often involves hitting moving targets or completing missions within a tight time limit, which require the player to not only have precise aim but also to make strategic decisions quickly. Moreover, as players progress, missions become more complex, with the need to account for factors like wind and distance to make accurate shots.

6. Strengths and weaknesses of the game:
Strengths of Sniper 3D include its satisfying sniping mechanics, diverse mission structure, and easy-to-learn controls that cater to both casual and dedicated players. However, the game's weaknesses may include a repetitive game loop for some players, and the presence of in-app purchases that can sometimes lean towards a "pay-to-win" model, which may put off players who prefer not to spend money.

7. What other games have been developed by the same developer:
Fun Games For Free, the developer behind Sniper 3D, has also developed a variety of other popular games including "Block Craft 3D: Building Simulator Games For Free," "Castle Crush: Epic Battle - Free Strategy Games," "War Machines: Tank Battle - Army & Military Games," and "Colorfy: Coloring Book for Adults," showcasing their ability to create engaging experiences across different game genres.