moat around a city, channel
壍 centers on a dug-out waterway for defense or flow: a moat surrounding a city or a channel directing water.
壍 combines 土 (earth) with 漸, which likely contributes the sound and the idea of gradual cutting or water flow, suggesting an earthwork channel or moat.
The bottom 土 is earth, and the top 漸 suggests a gradual flow. Picture earth gradually carved out to form a channel or a protective moat around a city.
For ザン, imagine a zany scene where a moat is dug in a zigzag pattern: zany -> ザン, and the zigzag channel surrounds the city.