fortress

fortress
/ˈfɔ:trɪs; `fɔrtrɪs/ n
castle or large fort; town strengthened against attack 城堡或大的碉堡; 加强防御能力的城镇
    attempts to capture this well-protected fortress 夺取这座坚固堡垒的多次进攻.

牛津英汉双解词典