The rare plants around Dinas Bran, a medieval castle and iron age hillfort above Llangollen and the Dee Valley, have been awarded national recognition.
Natural Resources Wales says important species of birds and butterflies "rely on the grassland and other habitats".
It is also home to upright chickweed, shepherd's cress and greater broomrap.
"The grasslands found here are scarce in lowland Britain," said Richard May, conservation officer for NRW.
"And Dinas Bran has the second largest area in Wales of one type of acid grassland."
The remains of Dinas Bran castle and hillfort date back to the occupation of the princes of Powys Fadog in the mid 13th century and they have been protected since 1957.
"Dinas Bran is hugely popular with walkers, attracting thousands of people a year to visit the iron age hillfort and medieval castle set in a spectacular landscape," added Mr May.
"By working in partnership with landowners, we look forward to helping to manage the site's special features so that we and future generations can continue to enjoy Dinas Bran's rich natural and historical heritage."
A "scarce" grassland around the ancient ruins of a Denbighshire castle has been protected as a site of special scientific interest.