Vishnu sahasranamam, slok83

Slok83:         Samaavartho  Nivrutthaatma Durjayo  Durathikramaha
                      Durlabho Durgamo Durgo Duraavaaso Duraariha

The equally enveloping Atma of Salvation,  which is the Impossible to win, the Impossible to overpower, the Fort- impossible to attain the entry, the celestial  region where it is impossible to reside physically, and the impossible  for enemies to conguer, is Sree Maha Vishnu.

This slok is refering to the place , where Vishnu resides.ie "VEIKUNTHA". The word
"DURGO" which means a Fort, is addressing a Structure  strategically designed.
In the previous slok also, few names like" Chathurvyuha, Chathuraathmaa "
are addressing, to this Fort like formation only. There is a  celestial arena, in which nothing can exist in any other form than "Energy "; whatever trys to supersede the order of that arena will be demolished. That is the palace of
"Salvation". Salvation is a state of "no return." Salvation means Nivrutthi.
That's why, that devine arena of Salvation (Liberation ) is called " Nivruttathma". Vishnu is Nivruttathma.
This is third time, this word "Nivruttathma" is mentioned. Every time, this name is associated with another word "Samaavartho ".

Vishnu is an equally spread Atma (a devine extrordinory circular field of attraction of unimaginable quantoms of energy forms, structured strategically) and  the Lord of that NivruttathAtma. That is VISHNU 's residence, the Ultimate goal.
VISHNU destroys the unrightuous entities ruthlessly.
VISHNU IS the Unparallel, incompetable POWER of thrilokas.

Om Vishnave namaha