  We have reinvestigated the A$^1\Pi$$(v=0)$ level of $^{13}$C$^{18}$O\ using
new high-resolution spectra obtained via multi-photon laser excitation as well
as with synchrotron-based Fourier-transform absorption spectroscopy of the
A$^1\Pi -\textrm{X}^1\Sigma^+$$(0, 0)$, e$^3\Sigma^--\textrm{X}^1\Sigma^+$$(1,
0)$, d$^3\Delta-\textrm{X}^1\Sigma^+$$(4, 0)$,
a$'^3\Sigma^+-\textrm{X}^1\Sigma^+$$(9, 0)$, and \atX$(11, 0)$ bands. In
addition, Fourier-transform emission spectroscopy in the visible range is
performed on the ${\rm B}^1\Sigma^+-{\rm A}^1\Pi$$(0, 0)$ band. Spectra of the
\BX$(0, 0)$ band are measured in order to tie information from the latter
emission data to the level structure of A$^1\Pi$$(v=0)$. The high pressures in
the absorption cell at the synchrotron and the high temperatures in the
emission discharge permitted monitoring of high rotational quantum levels in
A$^1\Pi$$(v=0)$ up to $J=43$. All information, in total over 900 spectral
lines, was included in an effective-Hamiltonian analysis of the A$^1\Pi(v=0,
J)$ levels that are directly perturbed by the e$\,^3\Sigma^-$$(v=1)$,
d$^3\!\Delta$$(v=4)$, a$'^3\Sigma^+$$(v=9)$, d$^3\!\Delta$$(v=0)$,
I$\,^1\Sigma^-$$(v=0, 1)$ close-lying levels and the e$\,^3\Sigma^-$$(v=0,2)$,
d$^3\!\Delta$$(v=3,5)$, a$'^3\Sigma^+$$(v=8,10)$ remote levels, as well being
indirectly influenced by the a$^3\Pi$$(v=10, 11)$ state. The influence of nine
further perturber levels and their interactions was investigated and are not
significant for reproducing the present experimental data. This analysis leads
to a much improved description in terms of molecular constants and interaction
parameters, compared to previous studies of the same energy region for other CO
isotopologues.
