Hello!
This sounds like an interesting concept to which I would like to add a suggestion:
Make it open for entering and leaving and do away with the must-be-odd number of players.
My reasoning is as follows: In the end, each turn only one player influences the history of the fortress as the other maps are all removed from the contest (but maybe their players want to continue them). So, even if there were 11 participants with the first turn, in the end, 10 of these participants will have contributed nothing to the map which is used in the second turn (they will have contributed their maps and their stories to the community, of course). In the third turn, no more than two players have left a mark upon the map before it starts, and only if it is not the same player winning both bouts.
Therefore, I think this game-style is very flexible about attachment as you don't have hard territories staked out. Instead, for everyone but one the game starts anew with each turn.
And allowing players joining and leaving may encourage participation, especially by insecure players or those with unstable schedules. In addition, players can join in, and later leave to concentrate on further developing their version of the map, branching off at any time of their choosing.
Yours,
Deathworks