Terrace rights of top floor owner

I am allottee of top floor in 1+3 stack of DDA SFS flats. I am 63 years old lady. It was locked as we were posted out of station. I left the terrace door unlocked to give access to lower floor owners for tanks etc. However, in my absence, my below & opposite neighbours abused my terrace, by holding drink parties,barbeque, throwing old junk like used toilet seats, etc. Also they grow huge plants, which dig their roots in the terrace bricks, causing damage to floor and walls of terrace.They neither keep the terrace clean, and expect me to visit periodically for cleanup, waterproofing, etc. Now there is seepage and huge chunks of plaster are falling in side of myflat, costing me Rs 3 lacs in repairs. I decided to make separate access through separate unlocked door for their antenna, dish, and tank area, for below owners, & keep main terrace locked.However they ganged up, saying terrace is common and I should leave it open for their general use. What are my rights, and what should I do in this context? ( Additional problem:-In fact the 2nd floor allottee just below me has made illegal alterations, and wants me to share lanter cost)