Pond sides

[LookoutTrout]

I haven't decided the finished size yet, I have about 12' x 20' to play with and thinking of something at least 6' diameter when finished. There's a small retaining wall above one side and another below on the opposite side, I thought about utilising the lower wall but I can see it (and the patio behind) subsiding as I dig down deeper so I'd rather keep it separate.
All subject to a cost estimate, I can't lose the soil elsewhere in the garden so I need to add the cost of skips onto the supplies. Unless it's fairly small, then the spoil will be used to create a water feature and hide the filter. I'm hoping I'll find I don't need to create a substantial foundation for the wall as that will makes thing much harder. The concrete/wall/painting paint will need finishing quickly as any rain wont drain away.