  ART$^2$ is a 3D multi-wavelength Monte Carlo radiative transfer (RT) code
that couples continuum and emission lines to track the propagation of photons
and their interactions with the interstellar medium (ISM). The previous version
of ART$^2$, which included continuum and Ly$\alpha$ line, has been extensively
applied to hydrodynamics simulations to study multi-band properties of galaxies
and ISM. Here, we describe new implementations of non-local thermodynamic
equilibrium RT of molecular and atomic fine structure emission lines, and the
parallelization of the code using a number of novel methods. The new ART$^2$
can efficiently and self-consistently produce a full spectrum that includes
both continuum and lines such as [CII], [NII], [OIII], Ly$\alpha$, and CO.
These essential features, together with the multi-phase ISM model and the
adaptive grid, make ART$^2$ a multi-purpose code to study multi-wavelength
properties of a wide range of astrophysical systems from planetary disks to
large-scale structures.
  To demonstrate the capability of the new ART$^2$, we applied it to two
hydrodynamics simulations: the zoom-in Milky Way Simulation to obtain
panchromatic properties of individual galaxies, and the large-scale
IllustrisTNG100 Simulation to obtain global properties such as the line
intensity mappings. These products are essential for a broad array of studies
such as the correlations between physical and panchromatic properties and their
evolution. By enabling direct comparison between numerical simulations and
multi-band observations, ART$^2$ provides a crucial theoretical framework for
the interpretations of existing observations, the plan for future surveys, and
the synergy between multi-band galaxy surveys and line intensity mappings.
Therefore, ART$^2$ is a powerful and versatile tool to bridge the gap between
theories and observations of cosmic structures.
