In the 1610s the Weimar Residenzschloss was partially rebuilt following a fire. The only surviving visual source for this reconstruction, an anonymous ground plan, has been attributed to Giovanni Bonalino, a master-builder from the Grisons. This article shows that the plan was in fact by Costantino de’ Servi, a Florentine polymath. The Italianate style of the ground plan has been noted in previous literature; here, some of de’ Servi’s sources of inspiration will be identified: Palazzo Marino in Milan, but also Litomyšl Castle in Bohemia, Richmond Palace in England and the local Saxon architectural tradition. Comparing archival sources held in Weimar and Florence thus allows this article to place the reconstruction of the Weimar Residenzschloss within its transnational, European context.