1. Introduction of the game:
"Castlevania: Symphony of the Night" is a timeless action-adventure platformer released by Konami in 1997. Famed for revitalizing the Castlevania series, it expertly blends gothic horror with RPG elements as players explore Dracula's enigmatic castle. Symphony of the Night abandons the level-by-level progression of earlier entries for a non-linear approach, allowing for exploration and discovery in true Metroidvania fashion.

2. All characters and levels of the game:
The game stars Alucard, the half-human son of Dracula, as the central protagonist, who is accompanied by other memorable characters such as the castle keeper, Richter Belmont, and the enigmatic Maria Renard. Throughout the vast, ever-changing castle, players unveil numerous levels, each with its distinctive Gothic atmosphere, from the marble-laced Entrance to the hauntingly beautiful Royal Chapel, the cavernous Underground Caverns, and the labyrinthine Clock Tower, culminating in the inverted castle, which doubles the exploration and challenge.

3. What attracts users most to the game:
Players are most attracted by Symphony of the Night's rich atmosphere, the depth of its gameplay, and its intricate level design. The RPG elements, such as character leveling, equipment customization, and spellcasting, add layers of strategy and personalization, making each playthrough unique. Its nonlinear design encourages thorough exploration and rewards player curiosity with secrets, powerful items, and hidden areas.

4. Why it is popular:
Its popularity stems from its successful amalgamation of action, exploration, and RPG mechanics, creating a rewarding and deeply engrossing experience. The game's intricate levels, haunting soundtrack, and striking sprite-based graphics have cemented its status as a cult classic, inspiring a genre of its own and leaving a lasting impact on game design.

5. When the game was released and how many versions are currently available:
Originally released in 1997 for the Sony PlayStation, "Castlevania: Symphony of the Night" has since seen multiple re-releases and is currently available on various platforms, including Xbox consoles, PlayStation 4, and mobile devices, as a part of collections or standalone downloads, often with enhancements and added features.

6. The most difficult part of the game:
One of the most challenging aspects of Symphony of the Night is navigating the inverted castle, a mirrored, more difficult version of the initial castle with stronger enemies and treacherous platforming segments. This, coupled with the need to discover and dispatch hidden bosses to obtain the true ending, provides a rigorous test of a player's skill and perseverance.

7. Strengths and weaknesses of the game:
"Symphony of the Night" boasts several strengths, including its atmospheric visual and auditory presentation, elegantly designed game world, and robust gameplay mechanics. However, some critics point out its occasional difficulty spikes and moments of backtracking as weaknesses, which, while integral to the genre, may deter some players.

8. What other games have been developed by the same developer:
Konami, the developer behind Symphony of the Night, is also known for other beloved franchises such as Metal Gear Solid, Silent Hill, Pro Evolution Soccer, and other Castlevania titles such as "Castlevania: Aria of Sorrow" and "Castlevania: Lords of Shadow". Their portfolio is diverse, touching on various genres across different gaming platforms.