  Massive stellar clusters have recently been hypothesised as candidates for
the acceleration of hadronic cosmic rays up to PeV energies. Previously, the
H.E.S.S. Collaboration has reported about very extended $\gamma$-ray emission
around Westerlund 1, a massive young stellar cluster in the Milky Way. In this
contribution we present an updated analysis that employs a new analysis
technique and is based on a much larger data set, allowing us to constrain
better the morphology and the energy spectrum of the emission. The analysis
technique used is a three-dimensional likelihood analysis, which is especially
well suited for largely extended sources. The origin of the $\gamma$-ray
emission will be discussed in light of multi-wavelength observations.
