The subsumption architecture is a layered mediator invented by Rodney Brooks for behaviour-based control of robots. The layers are minimally dependent and use minimal communication. We develop an object-oriented software design for the subsumption architecture, and demonstrate that each layer can be used as a slot for a set of plug-and-play components that implement different micro-strategies for achieving a particular goal. The software architecture, and the reuse of micro-strategy components, is validated by developing truck agents within the Truckin' simulation game. Guidelines for the development of specific layers and components of a subsumption architecture are also presented.