missile Silos allow you to build to things, IPMs and ABMs. IPMs are you used to destroy defense, but there is a catch, you can only send it so many systems away from your planet depending on the silo level. The higher the level, the more systems over you can hit, although i cant remember the exact forumula now. ABMs are for countering IPMs, one ABM will stop an IPM hitting your surface. ABMs are essential to any turtle! if you have a lot of D it is well worth investing in ABMs, so they aren't taken out.