[Alneyan]

The reason is to allow all nations to reach the Hill right as the clock starts ticking: 10 full turns is exactly what is needed to get to the Hill on turn 11, while waiting for turn 2 before attacking (there are nine provinces on the way).

If holding the Hill counted right from the start, nations having fliers or sneaking troops would have quite an edge (Caelum among them). It should be possible to get Teleport before turn 10, though that would require investing quite a bit in mages and/or having a Rainbow mage. Boots of flying or a flying Pretender should also do the trick.

At least, I believe it was my rationale when I initially set that rule for the first game.