Retaining can be on the boundaries, through the middle or at the front and rear of the site.
Retaining is required to create a level area to build on. Sloping sites may require retaining walls for ease of construction and to create a level pad.
If you don't have retaining, the earth will be benched or battered which makes the land or space unusable. Battering is the earth will take its natural form and fall at approximately 45 degrees from the flattest part of the pad. Benching is as it suggests – creating steps in the land to create small “benches” and is generally done in the cut side of the excavation.
Concrete sleepers are durable and expensive. They don't sit plumb and are ideal for post and piers construction types.
Block work is very accurate, durable and plumb. They're expensive and require a footing and are ideal for steel construction types.
Timber sleepers are cost effective but will need replacement after some time. They don't sit plumb and are ideal for post and pier construction types.