The idea of the map is a fight of two teams: one player defending the main island and the rest of them trying to invade it before the time expires. Well, i miscalculated the size of the buildings for each island, what ended up in a tiny map, but in a larger version of the map, there should be some tiny islands with towers surrounding the center island to help slow the invaders. the spiral way to the center of the island provides a large andvantage for the defender. Notice that the defender has (or will have in a larger map) enough resources for all the duration of the siege and only one possible direction of incoming attacks. Also there is one island right at the south with a high quantity of resources to supply the attackers (They should have only the essential to build a decent base and then build an outpost on the rich island), while providing some extra chance of slowing them (the defender could send some small army there just to buy time). How many attackers or how many time the fight should last is on the hands of the players to decide.
In red the passability of the areas (to emphasize that there is only one way to reach the defender's civ centre.