铜墙铁壁|tóng qiáng tiě bì
Meaning: an impenetrable defense; lit. copper wall, iron bastion
Original: “我们共护安全稳定,率先提出打击“三股势力”,坚决遏制毒品走私、网络犯罪、跨国有组织犯罪蔓延势头,联合举办反恐演习和边防行动,积极倡导政治解决国际和地区热点问题,构筑起守护地区和平安宁的铜墙铁壁。”
Source: Xi Jinping delivered a speech at the 21st Shanghai Cooperation Organization summit on September 17, 2021