Valmiki Ramayana Kishkindha Kanda Sarga 13 Shloka 20

द्रुमप्राकारसंवृतम् surrounded by a row of trees, एवं in this way, आश्रम् hermitage, तेषाम् their, प्रभावानां by virtue of their power of penance, सेन्द्रैः by those including Indra, सुरासुरैरपि gods and demons, सुदुराधर्षम् very difficult to enter.

'By virtue of the power of their penance, this hermitage surrounded by a row of trees is impenetrable even for demons and gods including Indra.