The chapel of Saint Basil is the only church in Romanesque style of West-Flanders.

Build in the first half of the 12th century by Derrick, Count of Alsace (1128-1168), the chapel is dedicated to Saint Basil the Great, a Greek doctor of the Church (+379) of whom a relic was brought back from Caesarea by Robert II of Jerusalem, Count of Flanders.

The chapel consists of two side naves and a central nave continued by the choir, which in turn is ended by a semi-circular apse.

The seated Madonna and Child or Sedes Sapientiae in the right nave is a wooden polychrome sculpture is dated back to early 14th century. The tympanum above the entrance linking the chapel and the annex is a 12th-century representation of the baptism of Saint Basil. Two remarkable wooden statues representJesus on the Cold Stone and the Pieta. Both sculptures, made around 1900, are highly revered and carried each year in the procession of the Holy Blood.

At the left of the choir, the chapel of Saint Yves was added in 1504. The relics of Saint Basil and of Charles the Good, Count of Flanders and who has been assassinated, are kept here. The black marble retable is presumed to be executed from drafts of Lancelot Blondeel.

The chapel of Saint Basil, the so-called chapel of the Precious Blood, was used by Count Derrick and his son Philip of Alsace while residing in the adjacent castle.

The lower chapel supports the upper chapel, originally built in Romanesque style, underwent in the course of centuries many significant transformations.
The title of basilica was bestowed to both chapels in 1923.
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The monumental staircase DE STEEGHERE that leads to the upper chapel was build in 1533 in Renaissance style ornamented on the outside with statues sculpted according to the drawings of Lancelot Blondeel.

The present building dates from the 19th century since the original was demolished during the French occupation.

The gilded bronze statues represent Archduchess Isabelle, Mary of Burgundy, Derrick and Philip of Alsace and, in the medallions, the Archdukes Albert and Maximilian of Austria, Margaret of York and Sybil of Anjou, wife of Derrick and mother of Philip of Alsace.