**Introduction to Call of Duty: Black Ops Zombies**

Call of Duty: Black Ops Zombies is an enthralling first-person shooter that builds upon the immense Call of Duty franchise, specifically branching out from the original Black Ops title. Developed with a focus on cooperative gameplay, the game plunges players into a dark and eerie world overrun by the undead. It combines the intensity of military combat with the horror of survival against hordes of zombies, providing a unique and adrenaline-pumping experience.

**All Characters and Levels of the Game**

The game features various characters from the Call of Duty universe, most notably the original crew - Tank Dempsey, Nikolai Belinski, Takeo Masaki, and Edward Richtofen - who have become fan favorites, each with their own backstory and quips. The levels, or maps, in Black Ops Zombies are varied, often inspired by locations from the main campaign or completely original settings. They range from the infamous 'Nacht der Untoten' to the moon-based 'Moon', each offering a different layout, set of obstacles, and Easter eggs that enhance the storyline and gameplay.

**What Attracts Users Most to the Game**

Players are primarily drawn to Call of Duty: Black Ops Zombies for its intense, cooperative gameplay and the challenge of surviving endless waves of zombies. The game's deep Easter eggs, secret songs, and hidden plotlines foster a profound culture of exploration and teamwork. The ability to upgrade weapons and strategize with various perks keeps the gameplay fresh and engrossing. Furthermore, the game's dark, atmospheric environments and attention to detail provide an immersive experience that keeps players coming back for more.

**Why It Is Popular**

The popularity of Call of Duty: Black Ops Zombies can be attributed to a number of factors. The thrill of battling zombies with friends is irresistible to many, and the game's incorporation of the familiar mechanics and polish of the wider Call of Duty series assures a quality gaming experience. Its popularity is also bolstered by the compelling storyline woven throughout the Zombies universe, intriguing both casual and hardcore gamers.

**Release Date and Available Versions**

Call of Duty: Black Ops Zombies was initially released as a game mode within Call of Duty: Black Ops on November 9, 2010. Following its success, it was adapted and released as a standalone mobile game for iOS and Android devices on December 1, 2011, and later for other platforms as separate content packs. As of my knowledge cutoff in early 2023, the franchise has continued releasing new versions and updates for different platforms with various map packs and expansion content.

**The Most Difficult Part of the Game**

One of the most difficult aspects of Call of Duty: Black Ops Zombies is the progression of difficulty with each wave of zombies. As players advance, the zombies become faster and more resilient, requiring quick reflexes, strategic thinking, and effective teamwork. High-level strategies and Easter egg quests also offer complex challenges that test even the most experienced players.

**Strengths and Weaknesses of the Game**

The game's strengths lie in its high replay value, compelling cooperative multiplayer experience, and its rich lore that captivates gamers. Its weaknesses might include occasional repetitiveness in gameplay and a steep learning curve for newcomers, which can be intimidating. The game's balance can also become disrupted when playing with players of varying experiences.

**Other Games Developed by the Same Developer**

Call of Duty: Black Ops Zombies was developed by Treyarch, known for their extensive work on the Call of Duty series. They are the primary developer of the Black Ops sub-series and have also contributed to titles like `Call of Duty: World at War` and `Call of Duty 3`. Additionally, they've worked on other games outside the Call of Duty universe, such as `Spider-Man` games released in the early 2000s.