(1567) PROMONTORY FORT on Trwyn Porth Dinllaen. This long narrow promontory, extending northwards for about II50 yds., is the most prominent headland on the north coast of Caernarvonshire. The sides rise steeply in cliffs about 50 ft. high, reaching 100 ft. along part of the E. side. The top is level or gently undulating, varying in width from 130 yds. to 300 yds. The sheltered bay to the E. is the only good natural harbour and landing place between Caernarvon and Bardsey.

Dinllaen is the ancient name of the fort, from which is derived that of the commote in which it stands.¹

Banks and ditches cut across the promontory 500 yds. from its N. tip, protecting an area of about 14 acres. The southernmost line of defence, much mutilated, is recognisable immediately W. of Ty-coch Inn. Only its western half is preserved (SH 27454157), for a length of about 130 ft. This consists of a flat-bottomed ditch some 36 ft. wide, with an inner side 5 ft. high, and a rounded counterscarp bank about 15ft. wide and 3 ft. high. There is now no sign of a bank on the inner lip of the ditch, but there are vague remains of one along the E. half of this defensive line, where the ditch and counterscarp have been obliterated by the cutting of a road leading down to the E. beach. Likewise at the extreme W. end the tail of the inner face of the ditch has been cut into and has disclosed a gravel make-up that may be the last remains of a bank.

A space averaging 200 ft. wide separates this rampart from the northern line of defence which lies N.W. of Ty-coch Inn and consists of a high bank of which only the E. half is preserved, for a length of about 120 ft. (SH 27504162). This is grassgrown, about 40 ft. wide at the base, 15ft. high on the outer (S.) side and 6 ft. high on the inner, with an inner quarry ditch 30 ft. wide. There is no outer ditch. Erosion shows this bank also to be of sand and gravel.
There are no remains of authentic antiquity within the defended area. On the E. side of the promontory, on the edge of the cliff immediately N. of the Ty-coch Inn (SH 27604164), is a vague square enclosure, about 36 ft. each way, with a low mound in the centre. Nine yards to the W. is an oval hollow, 15 ft. by 12 ft., with an entrance at the N. end. These were visible in 1918, but their purpose is unknown and their antiquity doubtful; they are probably the result of turfcutting.  A similar rectangular hollow just to the N., from which turf has recently been stripped, will look much the same when the grass has re-grown.

At the extreme N. end of the headland (SH 27734201) is a long rectangular building, 88 ft. by 20 ft., divided into four equal compartments, with grassgrown walls 2 ft. high; an observer in 1918 was informed locally that the remains were the foundations of cottages for men employed in the construction of the road and other works at the projected harbour.

Features that might be taken for a ditch and counterscarp bank on the E. side of the promontory at its extreme S. end (SH 27644103) are also not ancient. The ‘ditch’ is the last remnant of a sunken track, and the ‘countcrscarp bank’ is a modern field boundary.
Hemp (Arch. Camb., 1923, p. 148) records the occurrence of
fragments of flint ‘on the cliffs inside and outside the camp’.

Condition: greatly mutilated.

SH 27504160                                              1 x 57                                        31 N.E.