Many wireless sensor network applications rely on the availability of a collection service to route data packets towards a sink node. The service is typically accessed through well-defined interfaces so as to hide the details of its implementation. Providing for efficient network operation, however, often requires investigating the interplay between specific collection services and application-level algorithms. To enable a smooth evaluation of these mutual dependencies, we implemented a reference collection protocol, known as CTP, as a module for the Castalia wireless sensor networks simulator. Castalia is a well-known and widely used simulator but its standard distribution only provides for a basic collection module. By implementing a more advanced protocol like CTP we extend and improve the application scope of Castalia. In this report, we describe our implementation and present a study of the performance of CTP. All the software modules developed in the context of this work are available upon request from the authors.