Battlecruisers sit on a spectrum between cruisers and battleships rather than following one fixed technical formula. Speed, armour, and firepower form a triangle in which improving one quality usually costs weight or capability elsewhere. The useful balance depends on whether the ship will extend a battle fleet or conduct economic warfare independently. Britain could emphasize speed and guns because its battle fleet was already strong enough to contest sea control without battlecruiser reinforcement. Germany's smaller battle fleet gave its battlecruisers a stronger reason to carry protection and survive work alongside the line of battle.
Chapter · 4:02-7:03
Battlecruiser design begins with strategic purpose
The same speed-armour-firepower triangle produces different battlecruisers when a navy needs fleet reinforcement rather than independent economic warfare.