While compact steep spectrum (CSS) type sources only comprise approximately 20% of the extragalactic radio sources in flux limited samples (Fanti et al. 1990a), they comprise a much larger fraction, approximately 50%, of the extragalactic sources found to have very high Faraday rotation measures (RMs). We review what is currently known about the CSS type sources with high RMs and present new high resolution VLA1 observations of the CSS source 3C 318 at 3.6, 6 and 20 cm. These observations confirm the high RMs detected by Aizu et al. (1990) and reveal a change of sign in the RM across the source. The most likely source of the high RMs is a magnetized hot thermal gas around the radio source. The energy density of this gas and magnetic fields, however, is unlikely to be dense enough to confine the radio source. Our analysis suggests a fairly young source, less than 1 Myr old.