If you want to give your Citadel miniatures a bit more heft (say, to
prevent them from tipping over), glue some steel washers underneath
their bases!  There is a convenient cavity for, I conjecture, just
this purpose.  Since modern plastic miniatures weigh almost nothing, a
steel washer makes the center of gravity very low.

The added weight makes it feel much better to move the miniatures
around; they feel almost like chess pieces.