Your strongholds value increases by investing, collecting and buying.
You can upgrade shops with the architect at your villa in Monteriggioni, that will net you a fair % towards this. Also buying paintings, armour, and weapons will also increase the value.
Collecting feathers, codex pages, and seals also increases the value.
Start with looting chests then the fastest way to earn cash is to plough your money into all the shops till they are fully upgraded.
You can check in the book with your sister how much your value is, and you get bonuses for completing all shop upgrades for example.
You should get this around chapter 8 if you invest well.