  We present results on global very long baseline interferometry (VLBI)
observations at 327 MHz of eighteen compact steep-spectrum (CSS) and GHz-peaked
spectrum (GPS) radio sources from the 3C and the Peacock & Wall catalogues.
About 80 per cent of the sources have a 'double/triple' structure. The radio
emission at 327 MHz is dominated by steep-spectrum extended structures, while
compact regions become predominant at higher frequencies. As a consequence, we
could unambiguously detect the core region only in three sources, likely due to
self-absorption affecting its emission at this low frequency. Despite their low
surface brightness, lobes store the majority of the source energy budget, whose
correct estimate is a key ingredient in tackling the radio source evolution.
Low-frequency VLBI observations able to disentangle the lobe emission from that
of other regions are therefore the best way to infer the energetics of these
objects. Dynamical ages estimated from energy budget arguments provide values
between 2x10^3 and 5x10^4 yr, in agreement with the radiative ages estimated
from the fit of the integrated synchrotron spectrum, further supporting the
youth of these objects. A discrepancy between radiative and dynamical ages is
observed in a few sources where the integrated spectrum is dominated by
hotspots. In this case the radiative age likely represents the time spent by
the particles in these regions, rather than the source age.
