If you are buying a property built on a sloping site it needs to be inspected by a Chartered Surveyor very carefully. If you own such a home you need to check things over periodically.
Retaining walls between different levels in a garden restrain a great deal of weight and have a lot of damp earth banked up behind them, both features which can cause deterioration and collapse. Rebuilding a retaining wall can be very expensive!
You need to look at many aspects:is the wall vertical or showing signs of cracking and bowing?, does it have weep holes to allow groundwater to safely discharge?, was it built with a damp proof membrane, are the fence or railings fitted above it in a safe condition? what is the condition of the steps down?, is the patio level and does itincorporate suitable drainage arrangements? are the paving slabs clear of algae and safe/not slippy?, and so on.
If the wall has been covered with vegetation to “make it look nice” the plant growth may need to be cut away in places so the true condition can be established.
It is easy to get carried away by a beautiful garden, particularly on a lovely sunny day! For a moment though, take your rose tinted spectacles off and don’t forget to have a look in more detail at the garden.