Deuteronomy 20:12

And if it will make no peace with thee
Will not accept of terms of peace offered:

but will make war against thee;
come out and fight, or prepare to defend themselves: then thou shall besiege it; surround and block it up on all sides with their forces; the Jews say only on three sides, leaving one for any to flee and make their escape if they thought fit; (See Gill on Numbers 31:7).