Possible Clues:

• Impossible to enter
• Not admitting passage
• Impossible to understand
• Like Fort Knox
• A forest may become this: Peter and I blame last of woodmen for entanglement
• Impossible to pass through
• Meet April and Ben; they've become thick
• Giovanni came across half-hearted crowd, including writer hard to make out
• Fresh pine tree balm is thick
• Cryptic compiler's exercise learnt, be cryptic!