This ancient city had high walls and deep moats - easy to defend, hard to attack.
Tips
history
From 晁错's Han-dynasty memorial 《论贵粟疏》: "虽有高城深池，严法重刑，犹不能禁也" - even high walls, deep moats, strict laws and harsh penalties cannot stop hunger-driven rebellion. The phrase also appears in 《史记·礼书》, arguing that walls alone are not what truly secures a state.
memory
池 chí here is NOT "pond" but the older sense "moat" (the water encircling city walls). Visualize the four characters as a complete fortress: 高 high + 城 walls + 深 deep + 池 moat.
