Visited last summer on a lovely hot day. Drive through Llantwit Major and follow the road down to the car park at the sea shore. There is a shop here where you can buy the normal bucket/spade, chips, tea, ice cream etc. A nice little spot where the river runs into the sea and there are plenty of places to sit and ponder.
With ice cream in hand I headed along the coast, but to be honest I couldn't make anything out. The vegatation was waist high so I headed back and had a cuppa instead!
The Castle Ditches are part of some Iron Age defensive earthworks on the coast near Llantwit Major - some of the enclosure has probably fallen into the sea.
Marie Trevelyan mentions them in 'Folk lore and folk stories of Wales' (1909):
Among the places in South Glamorgan where the latest Beltane fires were kindled [was..] Llantwit Major between 1837 and 1840. The following information with reference to the Beltane fires was given me in these words:
" The fire was done in this way: Nine men would turn their pockets inside out, and see that every piece of money and all metals were off their persons. Then the men went into the nearest woods, and collected sticks of nine different kinds of trees. These were carried to the spot where the fire had to be built. There a circle was cut in the sod, and the sticks were set crosswise. All around the circle the people stood and watched the proceedings. One of the men would then take two bits of oak, and rub them together until a flame was kindled. This was applied to the sticks, and soon a large fire was made. Sometimes two fires were set up side by side. These fires, whether one or two, were called coelcerth, or bonfire. Round cakes of oatmeal and brown meal were split in four, and placed in a small flour-bag, and everybody present had to pick out a portion. The last bit in the bag fell to the lot of the bag-holder. Each person who chanced to pick up a piece of brown-meal cake was compelled to leap three times over the flames, or to run thrice between the two fires, by which means the people thought they were sure of a plentiful harvest. Shouts and screams of those who had to face the ordeal could be heard over so far, and those who chanced to pick the oatmeal portions sang and danced and clapped their hands in approval, as the holders of the brown bits leaped three times over the flames, or ran three times between the two fires. As a rule, no danger attended these curious celebrations, but occasionally somebody's clothes caught fire, which was quickly put out. The greatest fire of the year was the eve of May, or May 1, 2, or 3. The Midsummer Eve fire was more for the harvest. Very often a fire was built on the eve of November. The high ground near the Castle Ditches at Llantwit Major, in the Vale of Glamorgan, was a familiar spot for the Beltane on May 3 and on Midsummer Eve.