Very thick walls, moats, star fortress designs, barbed wire, modern bunker fortifications, etc. Many settlements, especially those in the outback, have ample defences constructed from need. Those that didn't are likely razed or paying heavy tributes. To siege a well-made fortress settlement is hard even with pieces of modern heavy artillery, as the more prosperous ones even have bunker shelters for bomber and artillery attacks. The most well-known fortress settlement is the city of Sydney, which is entirely surrounded by heavy fortifications. It's also likely that the settlement will actually possess more artillery than the attacking side, which will doom them from counter-battery fire.
