It might be worth leaving 1/2 your troops there and building Outposts where you can. Anyone attacking will have 30+ penalty to their battle advantage. You should be able to clean up even outnumbered 2:1 as long as your troops are elite, especially w/ archer retinues, which will probably stack wipe them in the first phase with 30+ advantage. And again, I'm not familiar w/ Sardinia's geography, but if you can build outposts, you would be able to even more decisively demolish anyone attacking you from sea.
Or, if you don't have the manpower to split your forces, and are spending hundreds of gold every war on embarkation, it would be a much better use of gold to just hire some elite mercenaries w/ archers and leave them on guard duty.
Coastline is all grasslands and hills, with the mountains only being on a couple interior baronies. There's a couple tiles where Hill Forts might be able to be made, but I think there's only 3 of 'em on the coastline and not all of them have castles. So best I could do is the Walls & Towers which need to be Level 5 before I get a defender advantage of +4, and that doesn't increase further.