Composite supernova remnants (SNRs) constitute a small subclass of 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 of 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....