Castalia is a Wireless Sensor Network simulator for early-phase algorithm/protocol testing built at the Networks and Pervasive Computing program of National ICT Australia. It supports realistic channel and radio models, a key element for accurate early-phase WSN simulation. It provides support for defining versatile physical processes. It also supports enhanced modeling of the sensing devices and other often-neglected attributes of a WSN such as node clock drift. Download and documentation at: http://castalia.npc.nicta.com.au.