The church is entered by the south porch, the inner doorway of which has a 3-centred arch within an opening which has lost its main arch and now has a lintel. This truncated archway has a broad hollow moulding at left within which there is a small carved figure in vestments, possibly representing St Peter.
The interior retains strong Perpendicular and late mediaeval character. The naves are of similar size, each of five major or ten minor bays; the naves are separated by an arcade of five four-centred stone arches on octagonal columns. The floor is of red and black quarry tiles. The roofs are a dominant feature; the major trusses have short pseudo-hammer beams with angel terminals (some missing), and the minor trusses have carved faces at the feet of the principals, recently painted. All trusses have high collar beams. In the north nave there are V struts above the collars, cusped to give trefoil and quatrefoil openings. The naves are now pewed as one, with C19 pews in three banks. The pew detailing is plain but there are simplified poppyheads to the ends. To the west of the north nave are two Jacobean box pews, within which a brass plate records the burial of John Jones of Scorlygen, gent, 1760. There is a C19 octagonal font near the south door.
The south nave now functions as the main nave, with the pulpit at its right side. This is octagonal, Jacobean (said to be about 1636), carved on all faces except the right where it stands against the wall, so perhaps in coming from the north nave it has been rebuilt. In the chancel, one step up from the nave, there are choirstalls similar to the nave pews. The chancel floor and that of the Lady Chapel in the north nave are paved in red and black quarry tiles, with encaustic features in the main chancel. There is a sanctuary step around the high altar with communion rails incorporating unusually tall closely set fluted balusters and no gate (said to come from the stairs landing at Plas Draw). The carved low-relief reredos is said to have the date 1636, and there is wall panelling each side and a pelican (1690) suspended above. The altar in the Lady Chapel is simpler and carries a 3-panel altarpiece.
Mediaeval stained glass is said to have been destroyed in the C17; some survived until replaced by C19 memorial windows. At south there is a window with 4 male saints , another with 4 female saints  and a third with four Old Testament leaders who built things: Noah with the ark; Abraham with the pyre; Solomon with the Temple and Nehemiah with the walls of Jerusalem; this window commemorates G F Lyster of Plas Issa . The 3-light window at north shows the Crucifixion, commemorating Catherine Williams of Glyn Arthur .
The church has a good collection of C18/19 Classical wall memorials, those at south to the family at Plas Draw from 1778 onwards. Those at north include a small one to the Rev David Hughes, rector and headmaster of Ruthin School  and a large Regency style one to the Rev John Jones, rector, of Plas-yn-llan [by Blayney of Chester, 1830]. Four earlier (C17/18) memorial stones, with lettering only, are set at low level in the north wall. Beside the south door is a table of charitable bequests to the poor of the parish.