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actually its more related to operation not general layout or final appearance of landscape.
if slope shall be high angle that shall increase running off water then using of wall is must.
just remember that at degree1:1 lost water by running off is% of added water.
it's depend on the design, but generally we can use the retaining walls if we have a very big differences in the level, or we need more flat areas in our gardens in order to use it as plaza, so every garden we can consider it a special case in design related to the slope and the main use for it .
There are actually no rule for that. It also depend on the difference in Leveling and if a slop cannot do the trick. it also depends on the taste in the design itself.
You can do a retaining wall for just 30 cm of difference between 2 levels though you can keep it as a slop.
It also depends on the function of the area. the retaining wall might be a good or bad idea.So more information would be helpful to have a better answer.