Computerized Adaptive Testing (CAT) is a field of research originating in psychometrics that has been adopted in the last years for formative and summative assessment activities in educational settings in general and in online learning in particular. While a variety of platforms is available for designing and deploying CAT the challenge of providing the flexibility in test and item design required for domain-specific assessment formats in education has hardly been addressed so far. The present article introduces a software architecture to fill this gap and enable the development of fully customizable CAT tools with respect to domain-specific item design and visualization as well as deployed CAT algorithms. A prototypical implementation of the architecture and a set of domain-specific item types are presented to demonstrate the feasibility of the proposed approach and outline future directions of development and research.