This is the 13906th most frequent Russian word.
оборонительных
"Defensive."
In this sentence, "оборонительным" is used as an adjective to describe "сооружениям," indicating structures related to defense.
Мы посетили музей, посвященный оборонительным сооружениям.
We visited a museum dedicated to defensive structures.
Here, "оборонительных" is describing "линий," referring to lines built for defense or protection.
Республика вложила средства в укрепление оборонительных линий.
The republic invested funds in strengthening defensive lines.
In this instance, "оборонительных" is used to describe "мер," indicating measures implemented for protection or defense.