noun
natural defenses; steep place
Refers to a naturally fortified location, such as a steep mountain or cliff, that provides defensive advantage. Often used in historical or geographical contexts.
その城は天険を利用して建てられた。
The castle was built taking advantage of the natural defenses.
この山道は天険の地として知られている。
This mountain path is known as a place of natural fortification.
要害 also means a strategic or fortified place, but can refer to man-made fortifications as well, while 天険 emphasizes natural terrain.
Compound of 天 (heaven, nature) and 険 (steep, precipitous). The exact historical derivation is uncertain, but the word has been used to describe naturally defensible terrain.