  We use photometric redshifts derived from new $u$-band through 4.5$\mu$m
Spitzer IRAC photometry in the 4.8\,deg$^2$ of the XMM-LSS field to construct
surface density maps in the redshift range 0.1-1.5. Our density maps show
evidence for large-scale structure in the form of filaments spanning several
tens of Mpc. Using these maps, we identify 339 overdensities that our simulated
lightcone analysis suggests are likely associated with dark matter haloes with
masses, $M_{\rm halo}$, log($M_{\rm halo}/M_{\odot})>$13.7. From this list of
overdensities we recover 43 of 70 known X-ray detected and spectroscopically
confirmed clusters. The missing X-ray clusters are largely at lower redshifts
and lower masses than our target log($M_{\rm halo}/M_{\odot})>$13.7. The bulk
of the overdensities are compact, but a quarter show extended morphologies
which include likely projection effects, clusters embedded in apparent
filaments as well as at least one potential cluster merger (at $z\sim1.28$).
The strongest overdensity in our highest redshift slice (at $z\sim1.5$) shows a
compact red galaxy core potentially implying a massive evolved cluster.
