Is there a readily-understandable antonym for "foundation"?

In addition to @Jason Bassford's good suggestions, capstone is a term that might work for you, depending on the context of what you want to say.

The term is often defined as a "high point", a "crowning achievement", or a sort of "finale".

It is also used as an architectural term, meaning "a stone on top of a wall or building".

That said, it is often used in academia, where students complete a "capstone project" at the end of their degrees. These projects are intended to bring together all of what the students have learned throughout the course of a full degree program, including "foundational" courses. There are many sources for this usage of capstone. Wikpedia aggregates a number of them and offers a synthetic description of the term's meaning in this context. Merriam-Webster, in its definition of capstone, offers examples of how the term is used, and includes a citation that mentions a student's "capstone project".

The site of the Harvard Business School's MS/MBA in Engineering Sciences offers an example of a capstone course. The description is lengthy, but here's an excerpt: "During Year 2, students..., during the January and spring terms, complete the Capstone course, in which they work in a small team to build and launch a new product...The MS/MBA: Engineering Sciences Capstone is an intensive project that requires teams of students to apply and integrate the skills they have learned across core disciplines developed in the program curriculum."

culmination

culmination NOUN

1 (usually in singular) The highest or climactic point of something, especially as attained after a long time.
‘the deal marked the culmination of years of negotiation’
LEXICO

Besides, don't they sound like they correspond? foundation ... culmination

The OP mentions a fence but not buildings. In a broader sense, pinnacle is sometimes used, although for a building it can be the apex of one section of the structure. A building usually, but not always, has a single connected foundation but may have several top points.

pinnacle NOUN

1 The most successful point; the culmination.
he had reached the pinnacle of his career

2.1 A small pointed turret built as an ornament on a roof.
Conical spires on top support pinnacles that enabled the towers to obtain the coveted height record.

Another example that works well in both senses (being built on the Acropolis) is

They came from the Parthenon, which marks the highest pinnacle of classical Doric architecture.

Frankly, I think capstone is the best choice, stylistically.

However, the word apotheosis is a great one and would fit here, in terms of formality.

It represents the pinnacle, perfection, or culmination of a process. For a staged process, such as the one that you describe, that is building up to a "highest" stage, this would be a good fit.