I thought I was going to go crazy: even after reducing the size of all the maps, the scene was so cumbersome that my computer wasn't able to render it!
To render the scene, I had to hide some characters and objects, and render several images with different parts of the scene. Then, I recontructed the final image by using masks in GIMP (cf. screenshots above).
Even like that, each "partial" render took more than 1h to be computed...

In fact, I think the most cumbersome parts are the lights in the scen, but removing some of them affected too much the global atmosphere...
Anyway, I hope you'll enjoy this final image!

Merry Christmas, everyone!