森严壁垒
strongly fortified; strictly defined line of demarcation between two schools of thought or two rival factions
早已～，更加众志成城。(毛泽东词《井冈山》)Already our defense is iron-clad,Now our wills unite like a fortress.(Mao Zedong’s poem Mount finggang)
森严壁垒sēn yán bì lěi
形容军事上戒备严密，也比喻界限划得非常严密。strongly fortified, closely guarded