Althusius refers to a few sources in his definition of the territory of the kingdom as the bounded place within which the law of the kingdom is exercised. I know that put like that alone it sounds pretty mundane: but it really is something of an innovation in the early seventeenth century, especially in the specific terms used. One of his references, as previously mentioned is Andreas Knichen. This is probably the major source, and the book as a whole is essential.
But he refers to several other sources. One is the line from the Roman jurist Pomponius in the Digest that “the territorium is the sum of the lands within the boundaries of a civitas; which some say is so named because the magistrate of a place has, within its boundaries, the right of terrifying, that is summoning (or expelling)”. (In classical Latin territorium is not straight-forwardly rendered as territory; it is much smaller scale, and this is an internal unit within the Empire, not a separate polity. But this definition is crucial for the 14th century jurists rereading Roman law in an entirely different context.)
There is also a reference to Udalricus Zasius, a lawyer working in the late 15th and early 16th centuries. I was surprised to find that there was a seven volume 1966 reprint of the 1550 Lyon edition in the British Library (who on earth was the audience for such a thing?) Zasius is building on Bartolus of Sassoferrato and Baldus de Ubaldis, who are treated in some detail in chapter 7 of my book. Like them, Zasius sees that jurisdiction is closely linked to a territorium, which is both the object of its rule and the thing that defines its extent. But Zasius—and this seems a fundamental development—ties supremacy to territory (superiotatis to territorium). I can’t recall a place where that happens before in those terms: it becomes crucial to Althusius’s definition.
Althusius also mentions Matthias Stephani, Tractatus de iurisdictione Liber II, which has an extensive treatment of iurisdictione territorii. This book is from 1610, and Althusius only refers to it in later editions of Politica. The British Library has a volume that has all the books of the Tractatus bound together, though they were issued separately. This chapter has an interesting discussion but Stephani regularly refers to Knichen, and draws on the standard references from the Digest, and Bartolus and Baldus, and I’m not sure that there is anything especially novel here.
There are a couple of other references to follow up – to Tiberius Decianus, Tractatus criminalis and Jacobi Bornitius, De majestate politica. I haven’t found the right texts or the references as yet. It can be frustrating because sometimes the indications are erroneous, sometimes the reference codes used can be hard to work out—not all these texts are paginated, for instance, so there are references to part/book/chapter/consilia, etc.—and because of the vagaries of early modern texts they are not always consistent. When there are multiple editions it’s often a case of finding the exact same one that was referenced, otherwise the numbering can be completely different. It’s pretty painstaking work… but it’s precisely because there are, relatively speaking, so few sources that I’m being pretty obsessive about tracking them down.