Lanka was surrounded by Trikut Parvat .This was very high ranges ,hence they were also knwn as Lamb Parvat.As Hanuman leapt into the air ,Mahendra Parvat thrust inside the earth , similarly when Hanuman landed on Lamb parvat , it became Lamba giri a small mountain due to thrust of Hanuman.
There was ocean around Lanka ,this gave it a Jaladurga formation ,difficult to access but since Lamba surrounded it , it could also qualify as Parvatdurga .
When leaping Hanuman increased his body dimension ,that was Mahima siddhi, Now to enter Lanka Hanuman took a miniature form ,this was Anima siddhi.
All rakshas are nishachar ,they are awake in the night and hence hanuman decided to enter Lanka in the night ,showing his enormous valour
Lanka was protected by Lankini.Every city has a 'gram' devata.Gramdevata protected cities and villages.Praying and offering oblations to the devata ensured protections to the villages and cities.
[ 2000 years after Kaliyuga started ,gram devatas disappeared from the earth and hence cities and villages expanded without borders and planning.5000 years after kaliyuga ,dieties governing rivers left the earth ,hence we could see drying of rivers , 10000 years after kaliyuga started Hari will leave the earth from temples.People will not worship in temples.]
Hanuman defeated Lankini with a fist strike . Lankini was gana devata from Durga devi's abode.She was a satvik and devotee additionally a woman, hence Hanuman just defeated her and not killed her. Simhika was also a woman but a evil woman hence Hanuman killed her.Hanuman's triumph over gramdevata shows he is the boss of all cities and villages.