With the growing aging population, age-related diseases have increased considerably over the years. In response to these, Ambient Assistive Living (AAL) systems are being developed and are continually evolving to enrich and support independent living. While most researchers investigate robust Activity Recognition (AR) techniques, this paper focuses on some of the architectural challenges of the AAL systems. This work proposes a system architecture that fuses varying software design patterns and integrates readily available hardware devices to create Wireless Sensor Networks (WSNs) for real-time applications. The system architecture brings together the Service-Oriented Architecture (SOA), semantic web technologies, and other methods to address some of the shortcomings of the preceding system implementations using off-the-shelf and open source components. In order to validate the proposed architecture, a prototype is developed and tested positively to recognize basic user activities in real time. The system provides a base that can be further extended in many areas of AAL systems, including composite AR.