roadblock, hurdle, obstacle, impediment, barrie, barricade, unbreachable, threshold, boundary, gate, more...

opening, gateway, passage, access, entrance

fence, wall, barbed wire, moat, electric fence, concrete barrier, chain link fence, wooden fence, iron gate, more...

brain, potential, major, effective, physical, natural, formidable, protective, placental, insurmountable, impassable

fence or wall,   gate or fence,   protective wall,   a wall,   natural obstacle,   tough obstacle,   it keeps apart,   large wall,   hadrian's wall,   concrete wall