防御 (defense, defensive) +
工事 (military works / earthworks). Standard military and historical vocabulary covering trenches, bunkers, walls, and pillboxes. Common collocates:
修建/
构筑/
加固 防御工事 (to build / construct / reinforce). Don't shorten -
防御工事 is the fixed phrase;
防御 alone is a verb/noun for 'to defend / defense' in general.