you can do the turrets but it would be kinda counter productive to need them in all LODs, just do the realy big ones (note turrets that are in more than LOD0 need to have there names ending in a for LOD0, b for LOD1, c for LOD2, ect...) you should have four LOD0 (the main one with full detains),
LOD1 (poly choped some small details, remove most of the small turrets),
LOD2 (probly only about 200 poly here at most, not turrets unless it's huge, spinning subobjects should be gone after here),
and LOD3 (a box or triangle)
if you want to make more go ahead, but this is how I do it,
oh and make it out of one subobject... or else

(im asuming you doing this right and
not throughing fourty or so primitives or spline objects together into a nonseemless mesh, there's been an upsurge in that sort of thing lately)