1. Introduction of the game:
Door Kickers: Action Squad is an intense side-scrolling shooter game that puts players in the shoes of an elite tactical SWAT team member. The game offers a unique blend of strategy, reflexes, and teamwork as players take on the role of highly-trained operatives tasked with neutralizing dangerous threats in various scenarios.

2. All characters and levels of the game:
In Door Kickers: Action Squad, players can choose from a diverse roster of characters, each with their own unique abilities and weapons. From adrenaline-fueled assault specialists to stealthy snipers, the game offers a variety of playstyles to suit different strategies. As for levels, the game features a wide range of challenging missions set in different environments, such as urban streets, buildings, and underground facilities.

3. What attracts users most to the game:
The game's combination of tactical decision-making and fast-paced action is a major draw for players. The ability to plan out each operation, strategically breach doors, coordinate with teammates, and adapt on the fly creates an immersive and thrilling experience. Additionally, the game's cooperative multiplayer mode allows players to team up with friends for even more intense and strategic gameplay.

4. Why it is popular:
Door Kickers: Action Squad has gained popularity due to its addictive gameplay and satisfying mechanics. The game perfectly balances planning and execution, allowing players to feel a sense of accomplishment as they execute successful operations. Its challenging difficulty curve also keeps players engaged, as they constantly strive to improve their tactical skills.

5. When the game was released and how many versions are currently available:
Door Kickers: Action Squad was initially released on October 24, 2018. As of now, there are two versions of the game available: the original version for PC, Xbox One, and PlayStation 4, and a newer version specifically designed for mobile devices.

6. The most difficult part of the game:
One of the most challenging aspects of Door Kickers: Action Squad is the high level of coordination required between players and their team members. Timing breaching maneuvers, coordinating attacks, and managing limited resources can be incredibly demanding, making every decision crucial to the success of the mission.

7. Strengths and weaknesses of the game:
Door Kickers: Action Squad's biggest strength lies in its compelling gameplay mechanics. The blend of strategy, reflexes, and teamwork creates a deeply satisfying experience for players. The game's retro pixel art style also adds to its charm. However, some players might find the game's difficulty to be a weakness, as it can be quite demanding, requiring several attempts to complete certain missions.

8. What other games have been developed by the same developer:
Door Kickers: Action Squad is developed by KillHouse Games, the same studio behind the highly acclaimed game Door Kickers. The original Door Kickers game focuses more on top-down tactical strategy, whereas Door Kickers: Action Squad puts more emphasis on fast-paced action and side-scrolling gameplay.