Innovation in residential energy systems drives research efforts towards novel building and district control strategies. Development and testing of such strategies requires advanced, interactive building and district simulators. This paper presents yards, an interactive simulator that combines the modelling capabilities of Modelica with a language-agnostic and cross-platform interface for controller development. yards offers modelling flexibility beyond that of existing tools, as any custom, user-provided building or district model can be implemented easily. A case study demonstrates how yards can be used to simulate a tiny cluster of buildings.