Romanesque architecture is a general term covering numerous variations of architectural style that flowered in Medieval Europe from about the late 10th century to around 1200.
There are, for example, French, English (aka Norman), German, Italian, Lombardy styles etc., many of which cross fertilised thanks to the itinerant lives of architects, masons, and craftsmen.
Even within countries, there are frequent regional differences (e.g. in Spain, between Catalan Romanesque churches and those of Aragon, Navarra and Castile-León). Modifications also occurred over time with improved techniques and changes in taste.
Most typically, Romanesque architecture is associated with church and church related buildings (e.g. monasteries, convents, hospices) although there are plenty of non-religious works also (e.g. castles, bridges, palaces).
How is it that Romanesque architecture spread throughout so much of Western Europe? Clearly, its international nature suggests that more trans-border travelling took place than is generally credited for in the Middle Ages. Pilgrimages were a fundamental reason for travel, but so too were the Crusades, commerce, royal marriages, alliances, wars, ecclesiastical studies and appointments, the expansion of monasteries, and the building opportunities offered to itinerant master masons, sculptors and craftsmen.
Romanesque entered Spain largely on the backs of travellers via two principal routes: first through Catalonia (Catalunya in Catalan; Cataluña in Castilian), and then through Aragón and Navarra from where it passed into Castile and León.
Castile-León, Navarra, Aragón.
Romanesque architecture did not reach the northern Christian kingdoms until roughly the second half of the 11th century. Following popular pilgrim passes through the Pyrenees en route to Santiago, it left a rich legacy in churches bridges and hospices from Aragón, through Navarra, Castile, León and into Galicia.


But how exactly did Romanesque architecture penetrate these northern kingdoms? The answer lies in the growing importance of the cult of St James (Santiago), and the influence of the Benedictine Abbey of Cluny (France) which popularised the pilgrimage and eased the journey by building roads and bridges and dotting the way with monasteries, churches and shelters.
Following the Benedictines came merchants and settlers from over the Pyrenees; French business immigrants even received special privileges from Spanish monarchs. Not for nothing was the route popularly known as the camino francés (“French Road”)**.
The passage of Benedictine influence in Spain was facilitated too by the close association between Cluny and northern Spain. Sancho the Great of Navarra (r. 1000-35), his son Ferdinand I of Castile-León (r. 1035-65) and grandson, Alfonso VI of Castile-León (r. 1065-1109), all established close ties with Cluny.
Sancho invited the Benedictines to reform the monasteries in his kingdom; Fernando bestowed on Cluny gold acquired from the parias paid by the small Muslim taifas of al-Andalus in return for protection; three of Alfonso’s wives were French, one of them being the niece of the abbot of Cluny.
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The years of greatest activity in the building of these churches, the 11th and 12th centuries, explains why Romanesque buildings in Spain are to be found mostly in the northern half of the peninsula.
Initially established along the pilgrim route to Santiago, they then spread down across the Duero valley, and onwards south, following the expansion of the Christian kingdoms of Castile-León and Aragón at the expense of Muslim al-Andalus.
However, with the south of the peninsula still firmly in the hands of the Moors (common term for the Muslims of al-Andalus), and with the invasions of the Almoravids in 1086 and the Almohades in 1145 reaffirming Muslim presence, Christian reconquest stalled roughly half way down the peninsula, until 1212 when large chunks of the south opened up following a major defeat of Almohad forces at Las Navas de Tolosa.
The fortress town of Avila and its neighbour, Segovia, conquered by Christians respectively in 1088 and 1085, are about as far south as we can go when talking of major Romanesque architecture in Spain.


Further south, there are very few churches, modest examples of the continuity of the medieval “church militant,” e. g. Santa Cruz in Baeza (late 13th century) or Mérida’s Co-Cathedral, Santa María la Mayor and Santa Eulalia (both late 13th century).

Gradually, however, Christian forces nibbled away at Moorish territory during the 12th century and in 1212 inflicted a major defeat on the Almohads at the Battle of Las Navas de Tolosa.
By now Romanesque was giving way to its architectural successor: Gothic. Nevertheless, the frontiers between Christians and Moors were subject to constant warfare and raids so that churches built to stamp Christian victories frequently adopted the solid structure of Romanesque architecture to the new Gothic taste. It’s often called transitional Romanesque Gothic.
Several such churches were built in Córdoba.following its conquest by Christian armies in 1236. Known as iglesias fernandinas (after Ferdinand III) who ordered their construction, they are hybrid Romanesque-Gothic with fortress-like qualities appropriate for unstable frontier areas, e.g. San Pablo, San Lorenzo. Another is the 13th century Church of Santa Cruz in Baeza, east of Córdoba.

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Let’s return now to the ancient kingdoms of northern Spain where there are hundreds of striking Romanesque churches in. One of the first and most beautiful buildings –despite later Gothic and Renaissance additions– is the Basilica of San Isidoro in León, founded by Ferdinand I and his Leonese wife, doña Sancha, in 1063 as a shrine for St Isidore, whose bones were transported there triumphantly from Muslim held Seville in 1063. St. Isidore was also intended as a pantheon for the kings of León.
Santiago de Compostela in Galicia is the most majestic of the Romanesque churches of Spain, but others too are impressive: the cathedral of Zamora, Santa María la Mayor of Toro, and the Old Cathedral of Salamanca (all three with a most unusual Byzantine style dome) to name a few.


Still, for many, it is the small village gems, visited perhaps on a hot afternoon in the company of the church custodian or priest, that spring to mind: San Martín in Frómista (Palencia) with its unusual pair of rounded towers and octagonal drum cupola (the church is all that is left of an earlier large Benedictine monastery), or San Pedro atop the decaying and remote mountain village of Caracena (Soria) with its superb arcaded gallery of seven arches (peculiar to the areas of Soria, Burgos and Segovia), or Andaluz (Soria), or Nafria la Llana with its robust little apse (S/W of Soria).


In these more isolated areas, away from urban noise and secular trappings, it is possible to glimpse something of the rhythm of the Medieval world and appreciate the fundamental role religion had in the lives of the people.
Unfortunately, however, many villages are now only faded shells of what they once were and their churches have fallen into disrepair. The young have migrated to the cities leaving only the old as tenuous links with the past.

For Romanesque art and sculpture in Spain, see Romanesque Art and Sculpture.
Sources:
Bango Torviso, Isidro El románico en España Madrid 1992
Barral I Altet, Xavier ed Art and Architecture of Spain Boston 1998
Dodds, Jerrilynn Architecture and Ideology in Early Medieval Spain Pennsylvania and London 1990
Eaude, Michael Catalonia: A Cultural History Oxford 2008
Gitlitz, David M & Davidson, Linda K The Pilgrimage Road to Santiago New York 2000.
Norman, Edward The House of God: Church Architecture, Style and History London 1990
White, Robert A River in Spain: Discovering the Duero Valley in Old Castile London, New York 1998
Image of walls of Avila by Pelayo2: http://en.wikipedia.org/wiki/%C3%81vila,_Spain
Image of San Pedro, Avila by Håkan Svensson: http://en.wikipedia.org/wiki/File:Avila_San_Pedro_View.jpg
León: San Isidoro by Luidger: http://en.wikipedia.org/wiki/Basilica_of_San_Isidoro
Image of Zamora cathedral:http://en.wikipedia.org/wiki/File:Catedral_de_Zamora_%28fachada_principal%292.JPG
Frómista by Santiperez: http://es.wikipedia.org/wiki/Iglesia_de_San_Mart%C3%ADn_de_Tours_%28Fr%C3%B3mista%29
Andaluz by Rowanwindwhistler: http://es.wikipedia.org/wiki/Andaluz_%28Soria%29
Santa Cruz, Baeza. By Anual: https://commons.wikimedia.org/wiki/File:Baeza-Iglesia_de_Santa_Cruz_FO-20110918.jpg
San Pablo, Córdoba: By Lancastermerrin88 – Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=8149332
Cathedral Mérida: By Elemaki – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=6577801