Does the idiom 高城深池 have a positive or negative connotation?
It is a neutral term. It describes a property of an object. Not commonly used in a metaphorical sense to describe a human characteristic
高城深池 is typically used to describe a defended city, base and of course, castle.
There are many idioms graphically describe objects with no metaphorical usage.
Example:
一望無際 (cannot see the edge) describe the view of a vast area - express: vastness
皮堅肉厚 (tough skin and thick muscle) literally describes a man or an animal with tough skin and thick muscle  - express: physically durable
水清山秀 (clear water and beautiful mountains) literally describes a place with clear water and beautiful mountains  - express: beautiful scenery
高城深池 literally describes a castle has high wall and deep moat - express: impenetrable defense