**1. Introduction of the Game**
Dota Underlords is a captivating auto-battler strategy game developed by Valve Corporation. It delves into the rich Dota universe, welcoming players to a world where tactical decision-making, unit placement, and synergizing team compositions pave the way to dominance. Players engage in battles against seven other opponents by fielding a lineup of heroes, each with unique abilities, and watch the combat unfold automatically, necessitating strategic foresight rather than twitch reflexes.

**2. All Characters and Levels of the Game**
The game features a diverse cast of characters, each originating from the iconic Dota 2 game. These heroes are sorted into various tiers, reflecting their rarity and power potential. They belong to different alliances, such as knights, mages, or warriors, adding another layer to strategic play through bonuses activated when fielding multiple heroes of the same alliance. Players level up by earning experience through matches or spending in-game currency, which allows them to place more characters into play and access higher-tier heroes.

**3. What Attracts Users Most to the Game**
The strategic depth combined with the ease of picking up the basics attracts users to Dota Underlords. The game offers a unique blend of tactics and RNG (random number generation), where every match feels different. The satisfaction of discovering potent hero combinations and the excitement of adapting strategies to counter opponents keeps players engaged. Moreover, the rich visual design and the presence of a familiar cast from the Dota world add to its allure.

**4. Why It Is Popular**
Dota Underlords' popularity stems from its association with the wildly successful Dota 2 and Valve's reputation for high-quality game development. It's accessible to newcomers yet deep enough for hardcore players, ensuring a wide appeal. Its free-to-play model with non-pay-to-win mechanics ensures a fair competitive environment. Additionally, its cross-platform functionality allows for a seamless experience on both mobile and desktop devices.

**5. When the Game Was Released and How Many Versions Are Currently Available**
Dota Underlords was officially released in February 2020 after a period in beta. Since its release, there have been several updates and seasonal content changes, but the core mechanics and game modes have largely remained consistent. The development team regularly releases patches to balance gameplay, introduce new items, and occasionally add heroes or features.

**6. The Most Difficult Part of the Game**
Mastering Dota Underlords requires understanding complex interaction between heroes, alliances, items, and positioning, which can be daunting. One of the most challenging aspects is the ability to adapt to an ever-changing “meta,” or the most effective strategies in the current game version. Additionally, high-level play demands quick decision-making and economy management to outmaneuver opponents.

**7. Strengths and Weaknesses of the Game**
Strengths of Dota Underlords include its deep strategic gameplay, appealing visuals, and regular updates that keep the game fresh. The game's ability to cater to both casual and competitive audiences also stands as a strong point.

However, its weaknesses lie in its steep learning curve that might deter newcomers, and the RNG elements can sometimes lead to frustrating experiences. Additionally, the lack of a tangible long-term progression system can impact player retention.

**8. What Other Games Have Been Developed by the Same Developer**
Valve Corporation, the game's developer, is renowned for creating several highly acclaimed game franchises. They are responsible for the creation of Half-Life, Counter-Strike, Portal, Team Fortress, Left 4 Dead, and more. They are also the developers of Dota 2, the direct inspiration for Dota Underlords, and operate the Steam platform, a leading digital distribution service for PC gaming.