Step 1: Introduction to the third level.
The third level refers to a mysterious, hidden level in the Grand Central Station that is discovered by the protagonist, Charley. It is not a regular part of the station and cannot be found on normal maps.
Step 2: Characteristics of the third level.
The third level is not a physical space but a temporal portal that connects Charley to a different time period, specifically to the 19th century. This hidden level offers a glimpse into an alternate reality, which Charley experiences firsthand.
Step 3: Symbolic Meaning.
The third level symbolizes escapism and the desire to break free from the constraints of modern life, offering a chance to experience a simpler time. It holds a deeper meaning in the context of time travel and the exploration of alternate realities.
Final Answer: \[ \boxed{\text{The third level refers to a hidden, mysterious level in the Grand Central Station that acts as a portal to the past, offering a time-traveling experience.}} \]