A verb appearing as G. osta- “to surround with walls, build up; fortify, protect; surround” in the Gnomish Lexicon (GL/63), based on the early root ᴱ√OSO as suggested by Christopher Tolkien (LT2A/Belegost; QL/s71).

Neo-Sindarin: Since ᴹ√OS “round, about” appears in Tolkien’s later writings, I would retain ᴺS. osta- for purposes of Neo-Sindarin, originally with the sense “to surround”, later more specifically “surround with walls” and by extension “fortify, protect”, probably due to the influence ost “(fortified) town”.