This paper presents a new edition and translation of the contract for the erection of the castle in Kórnik that was concluded in 1426 by and between a chancellor of the Poznań cathedral chapter, Mikołaj z Górki, owner of the Kórnik estate, and a Poznań carpenter Niklos (Nicholas). It is the oldest known document of the history of the first, medieval wooden castle. Written down in Latin, it has survived in the book of the Poznań Consistory. It has a special value as a source of information, but it is also unique from the point of view of diplomatics as the so-called chirograph (a document marked by its characteristic way in which it is written down and its manner of authentication). The contract is not only a first-rate source for research into the history of wooden and defensive architecture in late medieval Poland, but also, in view of the Polish words used to explain Latin terminology in the text, can be considered a memorial to the Polish language of the past. The four German words have a similar value, too.