The nave has a 4-bay arched-brace roof incorporating medieval timber. Beyond the simple plastered chancel arch the chancel has a 3-bay collar-beam roof with cusped raking struts and principals on corbels. The S side of the sanctuary has a C14 cinquefoiled tomb recess with a broken effigy of a woman of late C13 or early C14, traditionally said to be the wife of Dafydd ap Gruffydd, brother of Llewelyn Fawr. A 2-bay N chancel arcade, of 1894-5, has an octagonal pier and 2 orders of chamfer. A lower and narrower arch is further L in the nave. The N aisle has a restored late medieval 8½-bay arched-brace roof, on the S side resting on late C19 stone corbels. The SW vestry has a pointed doorway, which was the main entrance to the church before the W porch was added.
In early C20 reorganisation of the interior the chancel is effectively now only the 2 E bays, including the sanctuary in the E bay, and is separated from what is in ritual terms the nave by a low panelled screen. It incorporates panels re-used from family pews, including 2 inscribed 'WHM 1682', and another 'HG 1684', as well as vine trails and a quatrefoil frieze re-used from the cornice of the late-medieval chancel roof and noted by Sir Stephen Glynne c1839. The polygonal pulpit is an integral part of the dado and also incorporates re-used woodwork. The octagonal font has disc ornament to the facets, the E of which is inscribed '1661 INIR', and has an iron-bound cover. It is in the N aisle against the W tower wall where fragments of C14 sepulchral slabs were set up in 1904. The N aisle has 2 Decalogue boards and a Royal Arms of George I. The chancel reredos, a 1914-18 memorial, has canopied niches with figures of saints, in late medieval style.
Many windows have stained glass. In the chancel the E window of 1908 shows the Crucifixion and is by Joseph Bell & Son of Bristol. The SE window has medieval fragments; the middle window in the S wall is a war memorial with SS David and George and the SW window depicts Mary and Elizabeth, dated 1936 by Henry Dearle of Morris & Co of London. In the N aisle the E window shows the Ascension, of c1923, while the N window shows the presentation of Jesus at the Temple, of c1920. In the nave the S window shows the Annunciation, c1936. The W window, by Celtic Studios of Swansea of 1959, depicts the presentation of the royal decree to hold an Eisteddfod at Caerwys in 1523.
There are also numerous memorial tablets. The chief are a pair of contemporary classical wall tablets on the nave S wall. Both have slate inscription panels to freestone tablets with cherubs at the base. The tablet on the W side has a broken segmental pediment with cartouche and commemorates Thomas Lewis (d 1751), while the tablet on the E side has an open pediment and commemorates Samuel Mostyn (d 1751).