A page of History: visigoths in Peninsula.
Decadence of Roman Empire was almost a fact when its governors called many Gothic tribes in order to cross Danube river and defend Roman borders. In this way, Goths would obtain new lands. They were also pressed by other "barbarics", the "Huns" and they fought under their orders until they were recognized as a federate part of the Empire by the years of Theodosius (379-395). Visigothic -'Western Goths'- settlements would be taxfree since Goths offered their military services.
Esquema básico de las invasiones visigodas

Basic plan of Visigothic invasions
But peace was brief. Other germanic tribes often attacked from the north area. They were the reason of the Empire weakness and of its lack of coordination. At 410 A.D. Visigoths, leaded by Alaric, took Rome. Political strategy showed this new rulers that they should maintain the Roman Emperor: they tried to keep a "unity" that was in fact already disappeared.
Progress went on through the Gaul, where they founded Visigothic Kingdom of Tolosa, still a part of Roman Empire.
Little by little, they penetrated Roman Hispania in order to fight against other tribes already set in Iberian Peninsula: Suebi or Bagaudae. But year 507 -after Tolosian Visigoths defeat against Franks of Clodoveo (battle of Tolouse)- was the moment of the full Visigothic invasion of Hispania.
Once dominated the most part of peninsular territory, Visigoths set their capital in Toledo, a city strategically placed at the center of Hispania.
Problems of both cultures -Roman and Visigothic- was also religious. Visigoths became Arrianic: their doctrine could not be compatible with christianism -the latter already official in Roman Empire- because of the dogma. This problem increased until year 589: then, in Council of Toledo, King Recaredus converted himself to christianism. In this way, he got official support from Church and he also joined both societies -Hispanoroman and Visigothic- creating a true political unity .
He tried to consolidate his kingdom fighting against Suebi and Basques but he gave a great part of his power to a new born aristocracy. Members of this new social statement fought against each other, creating a situation of weakness and unstability. It provoked the entering of Moslems in Peninsula.
Mezquita de Córdoba
Mezquita de Córdoba
A page of History: Arabians in Peninsula.

Islamic conquest of the Peninsula was rapidly done. First facts were the work of "mercenary armies" from north-Africa, paid by Visigothic aristocrats who fought in civil conspiracies. Because of social situation of this new "Hispania" atacks were planned to get new lands. In this way, islamic armies began to conquer and govern cities that were disconcerted and had no strong governement to gave them shelter and rules. Soon they would become big urban towns well ruled and coordinated. Cultural, social and artistic superiority of Islamic civilization made that changes would be accepted by the most part of Peninsular population. Now, situation was closer to the one stablished after Rome conquest, eight years before.
Visigothic kingdoms and Peninsular aristocracies were gathering around small areas. At last they stood in the extreme north of Spain: the Cantabric one.
Preromanic architecture
In this phase, during Hispanogothics resistance, north of Peninsula was the scene for new artistic forms, different from the islamic ones imported by new conquerors. We cannot set common features to these ways of building, since each one shows individual ways of developping.
This characteristic will be shared by the whole old Europe until the age of Romanic. Then, different styles and building forms will be unified. Only Byzantine art kept in these years some uniformity and expanded itself up to the point that it was known in the Peninsula: byzantine constructions can be seen in Cartagena. Norsemen and Vikings attacks in the North of Europe increased these artistic lacks of definition. Because of that, this time would be called Age of Darkness in the center of Europe.
Scholars distinguish many sections for architectural Peninsular expressions:

Out of Iberian Peninsula
Constantino basilica
Constantino basilica, Saint Peter church. (Click to enlage)
Paleochristian art, developped in Roman Empire, is represented by constructions of catacombs and hidden graveyards in subterranean tunnels until Christianism legality (year 313). So we can see a model in Rome -major graveyard, of Domitila, Saint Callixtus-, but there are also interesting ones in Naples, Alexandria, Siracusa...
Later, as they did not need to hide themselves, first Christian churchs were built. They took a model in Roman basilica: a rectangular plan showing three naves with a wooden cover separated by columns. The central one was the wider and higher: it ended in an apse. Saint Marie the Major stands out in Rome as well as Saint Paul Outside the Walls or the original of Saint Peter, nowadays substituted. These features were ornamented in Orient with Hellenistic Mesopotamic or Syriac elements.
About 6th Century Byzantine art is a fact. It is composed of brick, cubic capital and domes, based on little triangular curved elements that let build a circular-plan dome on a square basis. Best instances can be visited in Constantinople: Saints Apostles, Saint Irene, Saints Serge and Baccus, and, of course, the masterpiece: Saint Sophie. Italie also constructed after these models: Saint Vidal in Ravena is an outstanding sample. Chronologically, this fashion would survive until 15th Century.
At last, we have to pay attention to the social and cultural Renaissance inspired in Europe by Charles the Great (8th-9th Centuries). The unity of a part of the Ancient Empire provoked the construction of churchs based on prior buildings. They will evolution to the Romanic principles as we can see in the churchs of Reichenau, Oberzel, Saint Phlibert of Grandlieu, and Aix-la-Chapelle.