**Introduction of the Game**
Welcome to the adrenaline-fueled world of DOOM, a groundbreaking first-person shooter that revolutionized the gaming industry. Developed by id Software and released in 1993, DOOM transports players to the Martian moons of Phobos and Deimos, where experiments with teleportation have opened a gateway to Hell itself. Players take on the role of a space marine, popularly known as the "Doomguy," fighting through hordes of demonic creatures in a bid to save humanity from a nightmarish onslaught.

**All Characters and Levels of the Game**
DOOM places players in the combat boots of the lone space marine, the unspoken hero at the center of the chaos. The enemies are a terrifying roster of demons, including the Imp, Cacodemon, and the fearsome Cyberdemon. Each episode consists of a series of levels, collectively known as 'Knee-Deep in the Dead,' 'The Shores of Hell,' and 'Inferno.' The fourth episode, 'Thy Flesh Consumed,' was added in the expanded version, "The Ultimate DOOM."

**What Attracts Users Most to the Game**
DOOM's relentless pace, visceral combat, and atmospheric level design make for an engrossing experience that still attracts gamers today. The game's innovations in 3D graphics, networked multiplayer, and user modifiability through WADs (Where's All the Data?) ensure a lasting legacy and replayability that are unmatched.

**Why It Is Popular**
DOOM's popularity can be attributed to its genre-defining gameplay, pioneering graphics, and the sheer thrill of battling through legions of demons. Its release coincided with the rise of PC gaming, capturing the imaginations of countless players. The ability to create and share custom levels has created a dedicated community and kept the game fresh and ever-evolving.

**When the Game Was Released and How Many Versions Are Currently Available**
DOOM was released on December 10, 1993. Over the years, multiple versions and sequels have been made available, including "The Ultimate DOOM," "DOOM II," "DOOM 64," the 2004 release "DOOM 3," 2016's reboot "DOOM," and its sequel "DOOM Eternal," among others.

**The Most Difficult Part of the Game**
Many players find the later levels particularly challenging, especially on the 'Ultra-Violence' and 'Nightmare' difficulty settings. "Thy Flesh Consumed" is notorious for its punishing level design and relentless enemy encounters, pushing players' skills to their limits.

**Strengths and Weaknesses of the Game**
DOOM's strengths lie in its fast-paced gameplay, mod community, and historical significance in the gaming industry. However, its weaknesses could include dated graphics by modern standards and a lack of complexity in the storyline, which for contemporary gamers might seem too simplistic.

**What Other Games Have Been Developed by the Same Developer**
id Software is known for creating genre-defining titles aside from DOOM, including the "Quake" series, "Wolfenstein 3D," and "Rage." The developer has played a crucial role in the advancement of real-time rendering technology in video games and continues to influence the first-person shooter genre.