As SoC complexity grows new methodologies and tools for system design and time-effective design space exploration are required. In this paper we introduce a tool called CASSE, what stands for CAmellia System-on-chip Simulation Environment. CASSE is a fast, flexible, and modular SystemC-based simulation environment which aims to be useful for design-space exploration and system-level design at different abstraction levels. The tool uses transaction-level modeling techniques for fast simulations and easy architectural modeling, and bridge the gap to system implementation by a progressive refinement approach. CASSE is being used in the European IST-2001-34410 CAMELLIA project, which focuses on the mapping of innovative smart imaging applications onto an existing video encoding architecture.