If I had to decide, I'd make the following rules for sites of famous battles.

- One side at least has to bring in 2500 soldiers (one side, becaue the battle very likely would be remembered if 200 guys would beat 3000 enemies)

- The battle has to be a Heroic Victory for one side OR a close victory, in which both sides lost at least 85% of their soldiers.

- Extraordinarily many men fought in this battle (at least 10'000 men alltogether)

- Special case: If the faction leader of one side OR at least 3 FM of one side die in this battle, at least 2'000 men have fought on both sides together, it can be a clear victory or an avarage one, and still get a site of a famous battle - marker

- The markers never disappear, since even today we remember great battles of ancient times, why should they forget them.

- In a radius of 3 Coordinates around a site of a famous battle, the faction who once had won this famous battle will get a morale-bonus if fightig at or near the same spot where their ancestors had achieved a great victory.