/ˈfɔːr.trəs/
a building or place that has been made stronger and protected against attack
a fortress town enclosed by four miles of ramparts
由四英里长的城墙围着的设防城镇
Fear of terrorist attack has turned the conference centre into a fortress.
由于害怕恐怖分子袭击，会议中心已变成了堡垒。