Lankhini was defeated

Sri Hanuma, after reaching the other shore, alighted on the mountain peak of Lamba and reduced his mountain-like body and obtained the normal stature.  After that Sri Hanuma observed the city of Lanka, having a resemblance of Amaravati, the capital City of Indra, from the top of the mountain.

Sri Hanuma neared the city of Lanka, which was decorated by moats filled with lotuses and water-lilies.  Lanka, which was surrounded by a golden boundary wall, was well protected, since the time of Seetha's abduction, by Rakshasas with horrifying voices roaming around and reached the northern gate.   

After observing Lanka Sri Hanuma thought that it was not possible for him to enter the city protected by cruel and strong rakshasas, with that normal appearance.  All those mighty Raakshasas, with extraordinary energy and great prowess, are to be cheated by him while engaged in the search of Seetha. 

He pondered over and decided for entering the city of Lanka in a small form at night to achieve that great deed successfully.  Then he decided for the sun to set.  After the sun had set, Sri Hanuma reduced his body at night with a size equal to that of a cat.

After reducing his size Sri Hanuma neared the city of Lanka.  Lankhini, the very soul and the guardian of that city, saw Sri Hanuma entering and rose up herself with a horrific appearance of face and eyes and obstructed him and questioned about his arrival into the Lanka.

Sri Hanuma replied that he would tell the truth provided she explains why she was threatening and obstructing him.  She explained that she wait for the orders of Ravana and was protecting that city of Lanka in all directions. 

अहम् राक्षसराजस्य रावणस्य महात्मनः |
आज्ञाप्रतीक्षा दुर्धर्षा रक्षामि नगरीमिमाम् ||  (Sundara Kanda 3rd Sarga 28th Sloka)

""I protect this city, being invincible. I wait for the orders of the king of demons Ravana, the great one."

She added that if he does not tell the truth death will occur to him. Sri Hanuma replied that after seeing Lanka, he would return to where he came from.  Observing Sri Hanuma’s evasive nature, Lankhini got angry and hit him with her palm.  Sri Hanuma roared and folded his fingers of left hand and hit her with his fist. Thinking that "She is a woman" he did not show much anger.

Lankhini fell on land immediately by that hit and with a falterting tone, without pride, acknowledges her defeat to Sri Hanuma.  She added that in earlier times she had been given a boon by Lord Brahma that whenever a Vanara conquers her with his might, then a fear will come for Rakshasas.

अहं तु नगरी लङ्का स्वयमेव प्लवङ्गम ||
निर्जिताहम् त्वया वीर विक्रमेण महाबल |  (Sundara Kanda 3rd Sarga 45th Sloka)

"O Vanara! I am myself the city of Lanka. O strong one! O courageous one! I am defeated by your strength."

She further said that with his appearance the time has come for her and Ordinance of Brahma is the truth and is immutable.  She finally concluded that the destruction has come for Ravana the evil minded king and to all Rakshasas due to Seetha and stepped aside for allowing his entry into Lanka.

Lankhini was saying that She herself is the city (अहं तु नगरी लङ्का स्वयमेव ) , ie., the soul of Lanka.  Earlier she said that she would wait for the orders from Ravana Himself.

She must be one type of GENIE, a spirit in Alladin Story, which will be hidden from vision but will rose up when occasion demands.  Demon King Ravana might have got control over her and kept guard over his Lanka. 

Lankhini earlier said that in earlier times she had been given a boon by Lord Brahma that whenever a Vanara conquers her with his might, then a fear will come for Rakshasas.  

Ravana might not have knowledge of this boon given by Lord Brahma.  Hence, he could not immediately recognised the threat to his Lanka and to him by Vanara.