  CTB 87 (G74.9+1.2) is an evolved supernova remnant (SNR) which hosts a
peculiar pulsar wind nebula (PWN). The X-ray peak is offset from that observed
in radio and lies towards the edge of the radio nebula. The putative pulsar,
CXOU~J201609.2+371110, was first resolved with \textit{Chandra} and is
surrounded by a compact and a more extended X-ray nebula. Here we use a deep
{\textit{XMM-Newton}} observation to examine the morphology and evolutionary
stage of the PWN and to search for thermal emission expected from a supernova
shell or reverse shock interaction with supernova ejecta. We do not find
evidence of thermal X-ray emission from the SNR and place an upper limit on the
electron density of 0.05~cm$^{-3}$ for a plasma temperature $kT\sim 0.8$ keV.
The morphology and spectral properties are consistent with a $\sim$20~kyr-old
relic PWN expanding into a stellar wind-blown bubble. We also present the first
X-ray spectral index map from the PWN and show that we can reproduce its
morphology by means of 2D axisymmetric relativistic hydrodynamical simulations.
