Step 1: Introduction to the third level.
The third level was a mysterious, hidden level in the Grand Central Station, where the protagonist, Charley, stumbled upon it. It was not visible to the regular commuters and could only be accessed in a very peculiar way.
Step 2: The setting of the third level.
The third level was situated in the Grand Central Station but was unlike any other regular platforms. It was a portal to the past, specifically to the 19th century, where Charley found himself transported to a different time period.
Step 3: Conclusion.
The third level was not just a physical place but a time portal where Charley experienced an alternative reality, making it one of the most intriguing aspects of the story.
Final Answer:
\[
\boxed{\text{The third level was a hidden, mysterious level in the Grand Central Station that acted as a time portal to the past, specifically to the 19th century.}}
\]