The Christian church discovered on an islet of the Ilissos River (known as Vatrahonisi), was a three-aisled basilica with a transverse aisle and narthex. To date, its length has been exposed up to 55m. Its chancel was covered by a dome, of which we do not know the exact form. Auxiliary areas of the basilica (exonarthex, atrium) are thought to extend to the west.

The basilica had a synthronon (chairs for the clergy) in the chancel, as well as luxurious decoration. The interiro walls were covered with revetments of Pentelic marble, while the floor was lined with high-quality mosaics. Fragments of this mosaic floor are on display at the Byzantine and Christian Museum in Athens. They depict geometrical motifs, vines with grapes, ivy leaves, birds and more. The Byzantine Museum also exhibits parts of the monument’s remarkable sculptural decoration.

Attached to the north aisle of the basilica is an underground space, square in plan, measuring 3,80 x 3,87m. which is believed to have been covered with a hemispherical dome. It is accessed from the interior of the nave by steps. Three Arcosolia (arched spaces intended for burial) were cut in three of of this structure’s walls, which were originally covered with Pentelic and Hymetian marble slabs. It constitutes a Martyrion, a place of burial and worship of Christian martyrs.

The existence of the Martyrion, has led scholars to identify the monument as a church, dedicated to the martyr Saint Leonidas. He was a bishop of Athens in the 3rd century A.D. By the end of the 12th century, Bishop Michail Choniates held that the Martyrion was near but not in the city (…a little before the city…). According to him it was also a ‘polyandrion’ (mass grave). It is thought that in this Martyrion the remains of the saint and seven saintly women who martyred with his were deposited. Their remains were transferred to Athens in the time of Constantine the Great. Based on this theory, the Martyrion was dated to the mid-4th century A.D., while the basilica was dated to the middle of the 5th century A.D. Recent research, however, supports the view that the architectural sculptures of the basilica of Ilissos date from the 6th century A.D.

The worship of Saint Leonidas is also associated with the magnificent basilica of Lechaion near Corinth, a church dedicated to the same saint. It was founded around the middle of the 5th century A.D.

The basilica of Ilissos appears to have been ruined during the Frankish rule of Athens (13th - 15th cen.). What remained completely disappeared either during one of the floods of Ilissos River or in 1778 when the Ottoman ruler of Athens, Hadji Ali Haseki, destroyed the ancient buildings in the area in order to build the city’s new fortified enclosure. In 1916-1917 the site was excavated by the archaeologist G. Sotiriou and the remains of this monument of great importance for the history of Early Christian Athens were once again brought to light.”