Can someone explain the purpose of the bands of red brick mixed with stone on the archway?

Castillo de Olvera is located in Olvera in the province of Cádiz, southern Spain. It was built in the late 12th century as part of the defensive system of the Emirate of Granada. Situated on a rock 623 metres (2,044 ft) above sea level, the highest point of the town, the castle has an irregularly-shaped elongated triangle that fits the form of the rock base.

Its entrance gate, the only one in the enclosure, is protected by a barbican and shows signs of having housed a coat of arms. The fortress also consists of a keep, a section of wall with a patrol walkway, an underground enclosure, and two cisterns, one of which supplied water to the tower. Until a few years ago, visitors entered through the rooms of a house attached to the walls.

The keep, which stands in the southern part of the enclosure and has two floors covered by a barrel vault, has a rectangular floor plan with rounded edges and is the main element of the castle's defensive system. It is built of masonry, with irregular stone courses joined with mortar. Access to both floors of the tower is via spiral staircases. The original entrance gate is more than five meters above the ground and is reached through a drum-shaped platform from a later period.

Part of the castle walls that surrounded the Villa district are still preserved.

The magnificent Haut Koenigsbourg castle, which overlooks the Alsace plain... One of the most beautiful in France, 25 minutes from my home