Translation

• secure as a city protected by a wall of metal and a moat of boiling water (idiom)
• well fortified
• invulnerable to attack

Synonyms

• 不衰, 坚不可摧, 坚固, 坚如磐石, 坚实, 坚牢, 壁垒森严, 巩固, 根深蒂固, 深厚, 深根固蒂, 牢不可破, 牢固, 稳固, 稳如泰山, 稳步, 结实, 金城汤池, 钢铁长城, 铁打江山, 铜墙铁壁, 长盛不衰