The so-called Temple of Diana — actually a Roman building of disputed function, possibly part of a library or imperial cult complex — stands in the lower section of the Jardins de la Fontaine, half-collapsed and roofless. The barrel-vaulted central hall survives; the rest is ruin.
You can wander through freely. The combination of weathered Roman stone, ivy, and the formal 18th-century garden around it is the most atmospheric corner of the park.