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In essence, to get the highest score, you should aim to:
1. Complete the map in the fewest number of game days.
2. Capture as many towns as possible by the end of the game.
3. Dig up the Grail. It is not necessary to place it in any town! It is enough to simply dig it up.
4. Destroy all enemy heroes. This will be, as you understand, the last action in the game. If your last move is to capture an enemy town, this town is counted in the calculation of the number of captured towns. Allied towns are not counted in the calculation. (There are none of these on the proposed map).