After crossing the sea and the hurdles therein Hanumaan arrived in Lankaa. He climbed on a Parvat to see the beauty of Lankaa. There he found many guards guarding Lankaa, so he thought that he would enter Lankaa in a tiny form and in the night.

When night fell, he took the form of a tiny fly, and tried to enter Lankaa, but there was a guard named Lankinee who was guarding Lankaa's main gate. She saw a fly going inside Lankaa, so she asked it - "Where are you going insulting me?" Hanumaan hit her with his fist. She fell down and bled through her mouth. She got up and greeted Hanumaan and said - "When Brahmaa Jee gave Var to Raavan, He told me the sign of the destruction of this Raakshas Kul, that "when you feel intense pain by a hit of a monkey, you should take it as the sign of death of Raavan. I am very lucky that I have got Darshan of Raam's messenger. Remember Koshal Raaj and enter the city." Thus Hanumaan entered Lankaa with the permission of the guard.

My Note
After this both Maanas and Raamaayan give different descriptions of Hanumaan's activities in Lankaa. Here most activities are listed according to Maanas.

Hanumaan went to see many palaces including Raavan's palace, but he couldn't see Seetaa anywhere. He was just wandering here and there, that he saw a palace with a temple nearby, in which there were many pictures of Raam's bow and arrow and several Tulasee plants. Hanumaan entered that palace wondering who was living there who had Tualsee plants in his house. As he entered the palace, Vibheeshan woke up saying "Raam, Raam". Hanumaan got very happy to hear Raam's name. 

Vibheeshan asked him who he was and what was his purpose to come there. Hanumaan told him that he was the servant of Raam and had come there to find about Seetaa. Vibheeshan told him the whereabouts of Seetaa. So Hanumaan went to Ashok Vaatikaa where Seetaa was sitting in a very unpleasant attire and mood. Many Raakshasee with fierce look were guarding Her. Hanumaan hid himself among the leaves of that tree and waited for the opportunity to introduce himself to Seetaa Jee.

The then Hanumaan saw Raavan coming there along with Mandodaree and other Raakshasee. He threatened Seetaa a lot saying that if She did not choose him as Her husband he will cut Her head. Hearing this Seetaa said - "Either I will have Raam's arms or this sword, this is my vow." Raavan immediately ran towards Her to cut Her head, but Mandodaree held him back to do so. Then Raavan told Raakshasee guards that if She did not obey him within a month, he would cut Her throat with that sword and went away.

My Note
It is written in Maanas , that Mandodaree stopped him by advising him ethically. But I doubt it that one Satee can face another Satee.  Anybody has any idea?

After Raavan had left Seetaa started weeping. There was a Raakshasee named Trijataa who was a great Bhakt of Raam, she said to Seetaa - "I saw a dream last night that one Vaanar has come here and burned the whole Lankaa, Raavan was riding on a donkey naked. So you don't worry, keep patience, and you will see my dream coming true." Seetaa asked her for fire so that She could immolate Herself, but Trijataa told Her that it was too late for it and went home. Other Raakshasee also went here and there.

Considering it a good time, Hanumaan dropped Raam's ring below. Seetaa picked that ring thinking it fire, but found it the ring of Raam. She was still doubting it, that Hanumaan started singing the glory of Raam. Then he came down the tree and stood before Jaanakee, told everything about Raam, Sugreev and their Vaanar army and asked Seetaa's welfare. 

Seetaa said - "You have already seen my condition. Tell Raam that if He doesn't come within a month, I will die." Then She asked - "You Vaanar are very short, how Raam will win Raavan with such army?" Then Hanumaan showed Her his huge form. Seeing it first Seetaa got frightened, then got convinced that if all Vaanar were like this then Raam would certainly get victory over Raavan.