I don't understand what crucial role that fountain plays there :( I know for sure that if you send fast units directly to the right and left of the Fortress and have helpers who, by the end of the first turn, will remove the slow units from them and, at the beginning of the second turn, will pass them on, then you can easily capture two fortresses in four turns. Moreover, in an ideal case, the left one can be captured on the third turn. I managed to do it. But I could only capture the right one on the 4th turn. Therefore, in the final playthrough, I didn't insist and captured both fortresses on the 4th turn simultaneously. But that nasty computer was far away :) I would have drowned it in the fountain for that :)