This is actually a great question and I don't want to discourage further discussion but in case no one gets to you here are some related discussions which may be of some use to you!

What were medieval European border crossings like?

When did borders become rigid?

Were Medieval borders between kingdoms as defined as today?

How did countries in the medieval age protect their borders?

The fact that these were the most relevant threads I could find and they only tangentially answer your question should tell you just how original you were here though! So applause is warranted for that alone :)

Okay, I am not qualified to actually give an answer here, but I can provide some literature in case you or anyone interested is able to read German (Please excuse the sloppy formatting of the citations):

• Pohl, Walter: "Herrschaft", in: Reallexikon der Germanischen Altertumskunde, Bd. 14, 2., völlig neu überarbeitete Auflage, Berlin/ New York 1999, S. 443-457.
  (Pohl looks at the definition of Herrschaft (hard to translate, best would be rule?) and this is related to what came before nation-states in the area you are describing.)

• Schneider, Jens: Punkte im Raum. Zur Bedeutung von Orten für die Ausbildung von Herrschaft. Berlin 2012.
  (Schneider with an interesting essay on how single points in space (like castles) were important for the establishment of rule. You can find this one online at: https://publikationen.uni-tuebingen.de/xmlui/handle/10900/47061 )

• Schneider, Jens: Raum und Grenze: "Vergleichende Überlegungen zur Entwicklung im mittelalterlichen Reich", in: Die Zukunft der Kartographie. Neue und nicht so neue epistemologische Krisen, hrsg. von Marion Picker, Veronique Maleval, Florent Gabaude, Bielefeld 2013, S. 177-197.
  (In this one Schneider specifically looks at the relationship between borders and space in the medieval Empire (Reich). He even focuses on the border between Germany and France. I just reread it and it is quite fascinating, but I don't feel that it can stand on it's own, but maybe not bad for an introduction since it is comparative in nature.)

A little anecdote stands out though: In the 13th century borders were beginning to emerge as a thing that people perceived as a kind of line between two realms (Altough there was no single definition or legal term for them. The latin CONFINES of late antiquity is what Schneider recommends as one term that appears in many sources, but TERMINUS and others are also important). Rudolf of Habsburg sent a comission in the argonne area to figure out the precise borders between France and Germany (the Empire). This was necessary since duke Bar-le-Duc was at odds with the French king Phillip the beautiful. The abbot of a local monastery had a dispute with the duke of Bar and asked the King for help in 1286. But the monastery in Beaulieu was in the diocese of Verdun, which had been part of Lotharingia in the 9th century. Thus it, and the duchy of Bar, should now be part of the Empire. So the duke protestet against the French military intervention. Shortly thereafter the Parliament in Paris passed a resolution that the duchy of Bar was part of the French kingdom. One year later in 1288 the Emperor send a commission to figure out the truth. This comission actually travelled the countryside and interviewed local lords about the question which realm they belonged to. Unsurprisingly enough the German commission declared the duchy a part of the Empire and the dispute would drag on for quite a while. p. 189-190.

• Türck, Verena: Beherrschter Raum und anerkannte Herrschaft. Friedrich I. Barbarossa und das Königreich Burgund, Ostfildern 2013 (Mittelalter-Forschungen 42).
  (Türck wrote a fascinating study on how Barbarossa integrated Burgundy into his realm and what this Burgundy actually looked like in peoples minds. Maybe not very interesting for you since her conclusion is that borders would have been more or less non-existent at a local level.)

• Vollrath, Hanna: "Geographischer Raum - politischer Raum - historischer Raum", in: Westmitteleuropa, Ostmitteleuropa: Vergleiche und Beziehungen; Festschrift für Ferdinand Seibt zum 65. Geburtstag, hrsg. von Winfried Eberhard, München 1992, s. 71-79.
  (Vollrath tells us how the relationship between space and the inter-personal networks ruling it. Consilium et auxilium and so on and so forth. The usual stuff.)

• Jaspert, Nikolas: Grenzen und Grenzräume im Mittelalter: Forschungen, Konzepte und Begriffe. Berlin 2007.
  (An overview over some central concepts like the Personenverbandsstaat (a state ruled through inter-connected networks of powerful people, etc.), rather bad if you ask me, but avaiable online at: http://archiv.ub.uni-heidelberg.de/volltextserver/16946/ )

• Hechberger, Werner: "Graf, Grafschaft", in: HRG, Bd. 2, Berlin, 2., völlig überarbeitete und erweiterte Auflage, 2012, S. 509-522.
  (A funny little essay that points out that the Empire was theoretically completely covered in duchies, but that these borders didn't really exist for the most part. Instead their rule manifested itself around points in space like castles or monasteries connected to the family. See Schneider (Punkte im Raum).)

That should suffice for a first introduction. Unfortunately I don't have anything in English.

edit:

It might be intersting to note how little detail about the partition of the Frankish Empire in the treaty of Verdun of 843 has come down to us over the centuries. You can find English translations of two of the most important historical sources on the Treaty here: http://oll.libertyfund.org/titles/2277 (I don't vouch for the accuracy, I have the Latin version of the chronicles here somewhere, but I was to lazy to look for it and just googled.) This is what all these nice little maps with perfect linear borders on Wikipedia are based on. As we can see the detail is excruciatingly insufficient if you want to increase the resolution of your map to the manor level. Where there aren't any natural borders we are quite literally unable to say where the border actually is, or whether there was any at all. But even natural borders were much less stable then you would think. Rivers would often change there course, so that cities could be on the Western side one century and on the eastern side the next. Sometimes they were islands in the river. I think the famous example here would be Breisach (Germany today) which is next to the river Rhein/ Rhine. It was also strategically important. Within Germany the border between Swabia and Bavaria or the dioceses of Augsburg and Constance was usually defined according to the rivers danube and Iller (Privilege of 1155 by Barbarossa for Hermann I. of Constance (Latin Version avaiable in the MGH)). Both rivers tended to change their course regulary though. Also intersting: The same Privilege of 1155 is the first descripition of the borders of the diocese of Constance in written form we have avaible. (Borders here termini) The descriptions are usually very vague when the diocese is concerned, but the personal holdings of the Bishop and the chapter were described in some detail. The only problem is that the text it was supposedly based on was outdated and may never have existed as such. Much of the Rest of the Privilege also doesn't hold much water. (edit2: On the Western side the border of the diocese of Constance was the border towards the French Kingdom)

see:

Die Urkunden Friedrichs I., hrsg. v . Heinrich Appelt u.a. (MGH Die Urkunden der Deutschen Könige und Kaiser [Friderici I. Diplomata] 10), Hannover 1975, DD F I 128.

RI I n. 1409, in: Regesta Imperii Online, URI: http://www.regesta-imperii.de/id/0854-07-22_1_0_1_1_0_3170_1409 (Abgerufen am 27.08.2014).

Büttner, Heinrich: Christentum und fränkischer Staat in Alemannien und Rätien während des 8. Jahrhunderts, in: Zeitschrift für schweizerische Kirchengeschichte 43 (1949), S. 132-150.

Derschka, Harald Rainer: Die Ministerialen des Hochstifes Konstanz, Sigmaringen 1999 (Vorträge und Forschungen 45).