Quite a lot of studies focus on the frontier as the defining constant and peculiar key to medieval Spain. There were no iron curtains or defensive lines and it was highly permeable. In some places like between the Catalan county of Pallars and the Lerida-Zaragoza Upper March around 1000 A.D., as Pierre Bonnassie shows, there was no strictly demarcated frontier at all, with Muslim and Christian settlements basically interspersed and people on both sides not really sure they actually lived, well, on the other side, so to speak (P. Bonnassie. La Catalogne du milieu du Xe à la fin du Xle siècle. Vol. 1, Toulouse, 1975, p. 118-128).

The frontier on both sides was considered a dangerous place of lawlessness, insecurity and wild beasts, a place of "great terror and trembling". The common trope in, say, Christian royal land grants in newly conquered lands is well, as everybody knows, this used to be a place where only bears and wild boars had roamed freely. It was commonly described as solitary, deserted, uninhabited and uninhabitable, ravaged by the back and forth of raids. Indeed, locus desertus, a wasteland, a permanently abandoned place, was an almost universal term for this no man's land for the Christians.

Things were always in flux on the frontier but there was a lot of opportunities as well. People crossed the frontier to trade - indeed, Bonnassie implies it was trading with the infidel that must have given the locals that reputation of particularly depraved and perverted folk that they enjoyed with their fellow Christians living in the hinterland. People crossed the frontier looking for war and glory. Political refugees routinely crossed the lines looking for asylum. If you ever read some nobleman was exiled from a Christian court you may bet it was in the Muslim lands that he spent his destierro before coming back into his liege's graces. Even the future conqueror of Toledo, king Alfonso VI, had been a guest of the local Muslim emir for almost a year. People crossed the frontier, sometimes for ever, looking for adventure and new beginnings. There was a surprisingly high number of elches, Christian converts to Islam, even in 1492, the very last year of Muslim rule in what remained of Al-Andalus. To quote Francisco Márquez Villanueva, 'Anyone who considered himself dishonored or felt that [he] had no future, even where this was the result of a failed or imposed marriage, had the option of going to Moorish territory and liarse la manta a la cabeza - a Spanish expression used to mean to tie a cloth around one's head - or, in other words, don a turban and convert to Islam, a term still used to denote radical or desperate behaviour' (F. Márquez Villanueva, On the Concept of Mudejarism, in: The Conversos and Moriscos in Late Medieval Spain and Beyond. Vol. 1, Departures and Change, ed. Kevin Ingram. Leiden, Boston, 2009, p. 33).

The frontier was also a place of peculiar institutions built around those opportunities. The alcaldía mayor dealt with disputes between Christian and Muslims, the alfaqueques' job was to rescue prisoners, an enaciado was a guy who had the trust of both parties, an adalid was a local expert in tracking and ambushes, and so on.

Physically, there was nothing to demarcate the border in the modern sense, of course. You wouldn't know it was there unless it happened to run along, say, a river - like it did along the Duero for most of the high Middle Ages in the West of the peninsula. What you would see was the areas on both sides of the border - a several days' ride on both sides earlier on in the history of the Reconquista - were noticeably less populated than the interior. You might notice the border areas had remarkably little in the way of political organisation and control as compared to the hinterland, with local military governors often setting up powerful dynasties and enjoying practical autonomy from the central power. If you were to cross the border, say, around the upper Ebro in the mid-9th century, you would have to travel through the lands of the local governor of the Upper March, Musa ibn Musa of the Banu Qasi dynasty who was considered 'the third king in Spain', on par with the Christian king in Asturias and the emir in Cordoba. At some point he even set up his half-brother as king in the Christian kingdom of Pamplona across the border and they joined forces to attack the emir.

As you rode to the border, you could probably see some of the castles that were built on both sides to control the area and deny freedom of movement to invading armies. But these were quite far apart and built at some strategic chokepoints so you could always skirt around them as an individual traveller. As you rode or walked on, you could see some of the many watchtowers closer to the actual border. These were built as sort of an early warning system and were not meant to stop travellers either. These still survive - at least in the many place-names in Spain. If you look at a map, you can almost see as the frontier moved south centuries ago, with names deriving from the Latin castrum (fort) or castellum (a smaller castle) gradually giving way to military place-names deriving from the Arabic qasr (castle) or qalat (fortress). The two Spanish words for a watchtower in use, almenara and atalaya, are both Arabic-derived and are, as Thomas F. Glick notes, 'a good example of an Arabic military term, representing an initially superior technology which was adopted at an early date in the Christian kingdoms' (Thomas F. Glick. [Islamic and Christian Spain in the Early Middle Ages] (http://libro.uca.edu/ics/emspain.htm), p. 62). You would see these but you wouldn't probably notice the actual border. At most places you wouldn't even probably realise it was there until after you crossed to the other side and seen strange watchtowers and, if you're lucky, a castle on the horizon.