Hanuman was looking at the beautiful Lanka built atop Thirikooda mountains. As he stood on top of thhe mountain, it rained flowers on him and he looked like a beautiful and decorated statue. He stood atop the mountain and saw the city, forests, palace and playground.
Lanka was architected by the great architect Vishwakarma. It was so beautiful that Hanuman was wonder struck. It was very well maintained by the demon king Ravana.
The gates of the city were guarded by poisonous snakes. Hanuman realised that the entire Vanara troop cannot enter easily into Lanka. He was trying to figure out an easy way to enter Lanka.
He also realised that he should shrink himself to the smallest of size and enter Lanka so that he is not sighted by the guards. He shrank himself and entered Lanka in the first portion of the night. He saw the city and he enjoyed it. Meanwhile, Moon rose up in the sky providing necessary light to Hanuman for his movement within the City