Assertion: keep's wall | /r/HasProperty | necessary
Facets:
value: from very early history
type: OTHER
value: for many cities
type: TRANSITIVE-OBJECT
value: often
type: DEGREE
Source sentences:
text: From very early history to modern times, defensive walls have often been necessary for cities to survive in an ever-changing world of invasion and conquest.
text: From very early history to modern times, walls have been a necessity for many cities.