Nowadays the software architecture of a system is often seen as a set of design decisions providing the rationale for the system design. When designing a software architecture multiple levels of design decisions need to be considered. For example, the service-based integration of heterogeneous platforms and the development of applications on top of those integration services requires high-level as well as technology-, domain-, and application-dependent architectural decisions. In this context, we performed a series of qualitative studies following a multi-method approach. First, we conducted a systematic literature review from which we derived a pattern language for platform integration featuring 40 patterns, as well as a pattern-based architectural decision model. Then, we performed interviews with 9 platform experts from 3 companies for revising the architectural knowledge captured by the pattern language and the decision model. Finally, we participated in a case study and observed the decision-making process to validate the results further. Our observations resulted in 1) a qualitatively validated, pattern-based architectural decision model and 2) a generalized model of the different levels and stages of architectural decision making for service-based platform integration.