From Liquipedia Warcraft Wiki
Upgrades are researchable improvements for heroes, units, and buildings. They can improve innate abilities or grant access to new ones. They can also improve stats of units or buildings like Animal War Training or the Improved Masonry. Upgrades for heroes are quite rare, as their stats and abilities usually get better as they gain experience and advance in levels.
|Improved Masonry||Nature's Blessing||Spiked Barricades|
|Advanced Masonry||Well Spring||Improved Spiked Barricades|
|Imbued Masonry||Reinforced Defenses|