Radiation shielding analysis is a crucial process in the spacecraft and space instrument development cycle. Simple tools such as SHIELDOSE-2 are used traditionally. With advances in computer and network technology and in energetic particle transport simulation codes, detailed Monte Carlo (M-C) simulation is no longer an approach reserved for research scientists but is widely becoming an important engineering tool. In this paper, we report on the main features of the multilayered shielding simulation software tool (MULASSIS) developed as part of the European Space Agency (ESA) activities in the Geant4 collaboration. It is based on the Geant4 M-C simulation toolkit developed by a large international collaboration lead by CERN. The software will be used as a general tool within ESA for radiation fluence, dose, and effects analysis. It has been integrated into the ESA SPENVIS system, thus, making it accessible to the wider space community over the World Wide Web.