1. Introduction of the game:
Dead Cells is an exhilarating rogue-lite, metroidvania-inspired action-platformer. Players navigate a sprawling, ever-changing castle infused with dark, Gothic elements and vibrant pixel-art aesthetics. You play as the Prisoner, a sentient mass of cells controlling a corpse in a grim dungeon, battling your way through hordes of creatures and navigating treacherous passageways to uncover the secrets lying within the imposing walls.

2. All characters and levels of the game:
Aside from the main character, the Prisoner, players encounter a host of enemies, each with unique patterns and attacks, as well as a few elusive NPCs that offer assistance or lore. The levels are procedurally generated, providing a fresh experience with every playthrough and include challenging biomes such as the Dilapidated Arboretum, the Morass of the Banished, and the Tower of the Hand of the King, among others. Each level comes with its distinct atmosphere, enemies, and secrets to discover.

3. What attracts users most to the game:
Players are most attracted to Dead Cells for its fast-paced combat, the thrill of exploration, and the rewarding feeling of mastering the game's mechanics. The combination of rogue-lite elements with metroidvania-style progression creates a compelling loop of gameplay that encourages repeated playthroughs, with new surprises and challenges each time.

4. Why it is popular:
Dead Cells is popular for its addictive gameplay, tight controls, and the satisfaction that comes from overcoming its tough but fair challenges. Its dynamic level design, coupled with a vast array of weapons and skills to experiment with, ensures that no two runs feel the same, catering to a wide range of playstyles and strategies.

5. When the game was released and how many versions are currently available:
Dead Cells was initially released in Early Access in 2017 and had its full release on August 7, 2018. Since then, the game has seen numerous updates and several DLC expansions, such as the "Rise of the Giant," "The Bad Seed," "Fatal Falls," and "The Queen and the Sea." These updates have added new areas, bosses, enemies, and items, thereby enhancing the game's replayability.

6. The most difficult part of the game:
The difficulty of Dead Cells ramps up considerably in the later stages, and once players defeat the final boss for the first time, they unlock the challenging "Boss Cell" modes. Each increased level of Boss Cell difficulty introduces tougher enemies and more stringent conditions, which even seasoned players will find to be a grueling test of their skills.

7. Strengths and weaknesses of the game:
Strengths include the game's fluid combat system, constant updates, vibrant visuals, and an engaging progression system. A potential weakness for some players might be the high difficulty curve, which can be unforgiving and might discourage less experienced players. Additionally, the reliance on procedural generation means some runs might feel less balanced than others.

8. What other games have been developed by the same developer:
Dead Cells is developed by Motion Twin, a French studio known for its cooperative web games like Die2Nite and Mush. More recently, Motion Twin has formed an offshoot studio called Evil Empire, which has taken over the ongoing development of Dead Cells and its future content updates, while Motion Twin explores new projects.