Upgrading shields is mainly about that guard boost stat which makes blocking way more efficient with your stamina. When you're looking at timing windows for blocking, the brass shield is a perfect example - going from +0 to +18 bumps the guard boost from 56 to 65, and that difference is huge during longer enemy combos. Watch the stamina bar carefully when blocking with a fresh shield, then compare after upgrades and you'll notice way less drain per hit.

The really interesting part is how guard boost improves every few upgrade levels specifically at +3, +5, and so on. Greatshields get even beefier benefits from upgrading, turning them into absolute walls especially when timing blocks against heavy attacks. Something worth noting about the barricade shield skill - pair it with an upgraded shield and you can actually hit 100 stability for those perfect zero-stamina blocks, which is crucial for surviving the nastier boss patterns.

Physical block stays the same through upgrades but that's fine since the stamina efficiency is what really matters for surviving those long attack chains. Just remember to prioritize your main weapon upgrades first, unless you're specifically building around shield combat and perfect block timing.