It is a two-tiered round burial mound with diameters of 23m (lower tier) and 18m (upper tier), and a height of 5m.
Literal
Diameter 23m (lower-tier) and 18m (upper-tier), height 5m [possessive-の] two-tier-style round-tumulus is.
An archaeological description. 円墳 (round burial mound/tumulus) is a specific historical-archaeological term. 二段式 (two-tiered style) describes the structure. 及び (and) is the formal conjunction. The parenthetical (下段)/(上段) clarify which measurement applies to which tier. である closes the description formally.