Serving an extensive rural parish in the Sussex Weald, it stands at the north end of its village on the site of an ancient pagan place of worship.

Long established local families have been important in the life of the church for centuries, as indicated by the extensive range of memorials and fittings in the building and its large churchyard.

The village got its name from the de Cahaignes family, one of whose ancestors is apparently commemorated by the rare 13th-century "heart shrine" in the chancel.

Nevertheless, the building still retains its original cruciform shape and its central tower topped by a landmark broach spire.

The church continues to play an active part in parish life, maintaining links to the local school and holding regular services.

Horsted Keynes is an ancient parish in the centre of Sussex, covering about 5,000 acres (2,020 ha) of heavily forested, mostly rural land which forms part of the Weald.

Nearly 480 acres (194 ha) was originally part of the ancient Forest of Anderida, and the soil consists of Hastings Sand and clay with several prominent sandstone ridges.

Sir William de Cahaignes held all the land in the parish,[2] and a Saxon noblewoman called Wulfgifa was the tenant of most of it.

St Giles' Church "may be one of the best examples of re-use of an old religious site" which was again a common practice in Sussex:[9] the present building, like its wooden and wattle and daub predecessors, stands within a stone circle which can still be seen in places,[8][9] and which probably contained a pagan temple.

[10] This may also explain its unusual orientation, northeast–southwest rather than the conventional east–west: the original pre-Christian structures on the site would have been aligned in this way so they would face the sunrise at the summer solstice.

[12] By this time, a stone-built Saxon church would have been in place; Ralph de Cahaignes possibly ordered its construction to replace the old wooden building.

[21] Spencer Slingsby Stallwood, a Reading-based architect, was commissioned to carry out his only work at a Sussex church at St Giles' in 1885,[22] in association with his colleague Joseph Morris.

[19] Changes inside included the removal of old box pews and a wooden gallery at the west end, which had latterly been used by church musicians.

[7] For many years the church was a peculier of the Archbishop of Canterbury rather than being held by the Bishop of Chichester, head of the local diocese.

Its nave is 18° north of east and the chancel 13°; this skewed construction recalls the leftward tilting of Christ's head on the cross.

[7] The church is built of locally hewn sandstone laid in large, rough blocks with wide joints.

[16] It is topped with a tall, slim, sharply pointed broach spire with an octagonal base and a covering of shingles.

[16] Tall slabs, up to 21 inches (53 cm) high, make up the jambs, which then support a pointed arch made up of ten voussoirs.

[10][28] The north wall was taken out in 1885 when the aisle was built; in its place a three-bay arcade was inserted, of which the easternmost bay reused 14th-century fabric.

[31] They are inscribed as follows: Inside and outside the church are a wide range of memorial tablets, slabs and stones commemorating members of locally important families such as the Wyat(t)s, Lightmakers and Piggot(t)s. Most date from between the 17th and 19th centuries.

[34] A study in 1846 described the effigy as being "of a fine grained oolite or sandstone" and taking the form of a cross-legged knight wearing 13th-century[9] armour (painted rather than carved on to the stone, and now invisible) and holding a sword but no shield.

k.b.m., referring to the initials of the donors and the Latin phrase "Ad Majorem Dei Gloriam" (Glory to God in the highest).

This area also incorporates some surrounding buildings such as St Giles' School: the churchyard does not cover the whole extent of the pagan site.

[42] Many of the gravestones date from the 17th and 18th centuries, including two gigantic stone slabs that were moved from the demolished south chapel to the wall outside the chancel.

Also found in the churchyard are some wood and stone "dead boards", an early type of grave marker that was easier to produce than a headstone.

His "seemingly very odd memorial"[9] by the southwest corner of the chancel[47] states that he was born on 30 December 1715 and died on 7 March 1715.

[9][34][47] The large parish has several hamlets, and in the late 19th and early 20th century there were two chapels of ease serving outlying areas.

After the latter stopped using it, Smythe opened the building out, extended it and turned it into a parish room for church activities.

[56] Children of the parish were originally educated in the chantry chapel of Marie de Bradehurst, which was part of the church.

[61] The ecclesiastical parish of Horsted Keynes covers a mostly rural area of Mid Sussex, much longer from north to south than it is wide.