This is the 14098th most frequent Russian word.
окопами
"Окопами" means "by/through the trenches."
The word 'окопами' is used here as an instrumental plural form of the noun 「окоп」, indicating the tool or means by which the defense was conducted.
Солдаты обороняли свои позиции с помощью окопами.
The soldiers defended their positions with the help of trenches.
In this sentence, 'окопами' describes the appearance of the frontline using the instrumental case to detail what it was filled with.
Линия фронта была усеяна глубокими окопами.
The frontline was dotted with deep trenches.
Here, 'окопами' is again used in the instrumental plural form, indicating that trenches are the method used for fortification.