Existing border levels defined by original (?) fencing
Flower beds. Soil. Flat or sloping gently
Pond. 10 cm below top of wall
Astronomy platform. Reasonably level. 30 - 50 cm below top of wall
Wall / bench. Level
The absolute height of the astronomy platform and wall are not important (as long as they are within 30 - 50 cm of each other).
Until the patio has been removed, it won't be clear at what height they'll be,
but the general principle should be to minimise the amount of material that is brought in and taken away.
All visible faces of wall should be good.
Not to scale. Wall ends when it is no longer required to hold soil at pond level away from fence.