Integrated circuit design has evolved to the stage where large, complex, analog and digital functionalities are implemented on a single chip or as an integrated chip set. Besides the mixed signal nature of the designs, the analog sections also include continuous-time and discrete-time components. Thus, for analysis of these integrated modules, all encompassing simulation capabilities are required that address, not only transient analysis of mixed analog-digital circuits, but frequency domain analysis as well. In this paper, we present mechanisms for multilevel and mixed-domain simulation of analog systems tied in with mixed analog-digital simulation. We then describe the implementation in the form of an open-ended and expandable simulation framework, iMACSIM. A simulation backplane is used to provide a general event-processing and scheduling framework that ties together the various algorithms necessary for simulation of various classes of circuits in different analysis regimes.