天塹

天堑
tiānqiàn
natural moat

CEDICT

  1. Heavenly Barrier (in reference to the Yangtze)
  2. natural moat

Wiktionary

天然的河海險要地。

重編國語辭典

Entry Composition

1 天 tiān
  1. day
  2. sky
  3. heaven
2 塹 qiàn (bound form) moat; chasm