  Composite supernova remnants (SNRs) constitute a small subclass of the
remnants of massive stellar explosions where non-thermal radiation is observed
from both the expanding shell-like shock front and from a pulsar wind nebula
(PWN) located inside of the SNR. These systems represent a unique evolutionary
phase of SNRs where observations in the radio, X-ray, and $\gamma$-ray regimes
allow the study of the co-evolution of both these energetic phenomena. In this
article, we report results from observations of the shell-type SNR G15.4+0.1
performed with the High Energy Stereoscopic System (H.E.S.S.) and XMM-Newton. A
compact TeV $\gamma$-ray source, HESSJ1818-154, located in the center and
contained within the shell of G15.4+0.1 is detected by H.E.S.S. and featurs a
spectrum best represented by a power-law model with a spectral index of $-2.3
\pm 0.3_{stat} \pm 0.2_{sys}$ and an integral flux of
F$(>$0.42$\,\mathrm{TeV}$)=($0.9 \pm 0.3_{\mathrm{stat}} \pm
0.2_{\mathrm{sys}}) \times 10^{-12}$cm$^{-2}$s$^{-1}$. Furthermore, a recent
observation with XMM-Newton reveals extended X-ray emission strongly peaked in
the center of G15.4+0.1. The X-ray source shows indications of an
energy-dependent morphology featuring a compact core at energies above 4 keV
and more extended emission that fills the entire region within the SNR at lower
energies. Together, the X-ray and VHE $\gamma$-ray emission provide strong
evidence of a PWN located inside the shell of G15.4+0.1, and this SNR can
therefore be classified as a \emph{composite} based on these observations. The
radio, X-ray, and $\gamma$-ray emission from the PWN is compatible with a
one-zone leptonic model that requires a low average magnetic field inside the
emission region. An unambiguous counterpart to the putative pulsar, which is
thought to power the PWN, has been detected neither in radio nor in X-ray
observations of G15.4+0.1.
