trench, dugout, air raid shelter
壕 centers on a dug-out space in the ground: a trench, dugout, or air raid shelter. All meanings share the idea of a protective or functional excavation.
壕 combines 土 (earth, ground) with 豪, which likely contributes the sound コウ/ゴウ and carries a sense of something large or imposing. The character thus suggests a large earthwork or excavation.
The earth radical 土 on the left shows this is about the ground. The right side 豪 looks like a grand, imposing structure. Picture a grand trench or dugout carved into the earth.
For コウ, imagine a deep trench that echoes with a low, hollow sound: cove -> コウ. The hollow cove-like sound helps you remember the reading.