  The expansion of Galactic HII regions can trigger the formation of a new
generation of stars. However, little is know about the physical conditions that
prevail in these regions. We study the physical conditions that prevail in
specific zones towards expanding HII regions that trace representative media
such as the photodissociation region, the ionized region, and condensations
with and without ongoing star formation. We use the SPIRE Fourier Transform
Spectrometer (FTS) on board $Herschel$ to observe the HII region RCW 120.
Continuum and lines are observed in the $190-670\,\mu$m range. Line intensities
and line ratios are obtained and used as physical diagnostics of the gas. We
used the Meudon PDR code and the RADEX code to derive the gas density and the
radiation field at nine distinct positions including the PDR surface and
regions with and without star-formation activity. For the different regions we
detect the atomic lines [NII] at $205\,\mu$m and [CI] at $370$ and $609\,\mu$m,
the $^{12}{\rm CO}$ ladder between the $J=4$ and $J=13$ levels and the
$^{13}{\rm CO}$ ladder between the $J=5$ and $J=14$ levels, as well as CH$ ^{+}
$ in absorption. We find gas temperatures in the range $45-250\,$K for
densities of $10^4-10^6\,{\rm cm}^{-3}$, and a high column density on the order
of $N_{{\rm H}}\sim10^{22}\,{\rm cm}^{-2}$ that is in agreement with dust
analysis. The ubiquitousness of the atomic and CH$ ^{+} $ emission suggests the
presence of a low-density PDR throughout RCW 120. High-excitation lines of CO
indicate the presence of irradiated dense structures or small dense clumps
containing young stellar objects, while we also find a less dense medium
($N_{{\rm H}}\sim10^{20}\,{\rm cm}^{-2}$) with high temperatures ($80-200\,$K).
