[Turret]

A tank would be protected from threats, in the operational roles it would be demanded to fulfil.

For instance, if a Leopard was in the 44-49 range in frontal protection, it would be protected against the threats of its era, 100, 115 cannons, even RPG's, etc.

Even possibly, the early 125 rounds.

I'm making a point. A tank must be protected sufficiently to operate, to deliver, deter, and to reply fire, protected.

Bullet-proof, auto-cannon proof, even up to tank guns? That would be an MBT or heavy tank, to my mind.