1. Introduction of the game:
Tank.io is a thrilling and fast-paced multiplayer online tank battle game that merges strategy, shooting, and real-time combat. Set in simplistic yet engaging arenas, players maneuver their tanks, destroy obstacles, and eliminate opponents in an attempt to survive and dominate the battlefield. Featuring intuitive controls and a competitive gameplay experience, Tank.io appeals to gamers of all ages seeking quick, action-packed battles with a tactical edge.

2. All characters and levels of the game:
In Tank.io, "characters" are represented by a variety of tanks, each with unique abilities, weaponry, and upgrade paths. Players begin with a basic tank and, as they gain experience points by eliminating adversaries and objects, they unlock new, more powerful models with diverse attributes. The game features multiple levels which offer increasing complexity and difficulty, challenging players to adapt their strategies and improve their precision to survive longer and claim victory.

3. What attracts users most to the game:
Users are predominantly drawn to Tank.io for its simple yet addictive gameplay mechanics, the satisfaction of leveling up and customizing their tanks, and the rush of competitive multiplayer action. Instant join-in matches allow for minimal downtime, and the game's leaderboard system induces a sense of competition that keeps players engaged, striving to achieve higher ranks.

4. Why it is popular:
Tank.io's popularity can be attributed to its accessible gameplay that's easy to learn but hard to master. The game's appeal also lies in its short, intense game sessions and the immediate gratification of destroying other tanks. Moreover, the fact that it is playable on various devices and platforms ensures a broad reach and connectivity among friends and the gaming community.

5. The most difficult part of the game:
The most challenging aspect of Tank.io often involves managing multiple opponents simultaneously while navigating increasingly complex levels. As the player advances, they must not only refine their attack strategies but also master evasion and resource management, all while the gameplay becomes faster and the stakes higher.

6. Strengths and weaknesses of the game:
Strengths of Tank.io include its seamless multiplayer experience, diverse tank upgrades, and a level of depth through strategic gameplay choices. On the flip side, a potential weakness could be a lack of variability in game modes that may lead to repetitive gameplay. Additionally, the simplicity of graphics might not appeal to players looking for a more visually immersive experience.