天堑的英文：
natural moat

参考例句：

The natural barriers for defense are insurmountable
天堑难渡

A bridge will fly to span the north and south, turning a deep chasm into a thoroughfare.
一桥飞架南北,天堑变通途

natural是什么意思：
adj. 自然的；天然的；不做作的；天生的；原始的；有血缘关系的
n. 天然物；生就具有特定才能的人

natural | natural sign
白键|本位记号

The method is a natural.
这方法一定灵。

Is there necessity in nature?
自然界是否存在必然性？

It is in the nature of lightningrods.
它具有避雷针的性质。

natural language
自然语言(指人类语言集团的本族语,与世界语或计算机语言等人造语言相对)

moat是什么意思：
n. 壕沟,护城河
vt. 将…围以壕沟

A ledge between the parapet and the moat in a fortification.
搁板在堡垒中护城河与护墙之间的搁板

The medieval town is fortified with a high wall and a deep moat.
这座中世纪的城市用高墙深沟来加强防御。

The medieval town was fortified with a high wall and a deep moat.
这座中世纪的城市周围建有城墙和护城河。

A moat separates the animals in the zoo from the spectators.
一条深沟把动物园里的动物和观众隔开

The strand of that island was its first boundary wall the Seine its first moat.
这小岛的河滩就是巴黎最早的城廓，塞纳河就是它最早的沟堑。