I've found out that the smaller the fort is the easier it is to defend it. There to much wall to cover in a star fort and you can't defend all of them even with a full stack. There so many spot where you can't fire from. I prefer normal fort where you can spare a few unit to garrison the inside building for a last stand and still man all the wall decently. I hope they will improve the star fort in future patch because right now they are indeed a waist of income.