Well, its basically just a medieval version of american football.
You have to get your goat into the endzone to score a point. You can also shoot down the poles to score points and you have to keep your archer alive long enough to score two points with him. You can use your trolls to protect him and you have to do all this without letting the computer get the sheep into your endzone!
The main problem here is that the AI doesn't know how to play. The default AI and those few triggers you used simply aren't enough to make the game interesting. The AI control triggers would need a little more work here to make the AI play a reasonable game. (A custom made AI file would be even better of course - some randomised herdable plans etc. would make for a much more interesting game than a couple of 'move to point' trigger effects.)
Also, I personally would have preferred it if you had designed a 'real' end zone and let the game continue with a new 'kick-off' every time a team scores. Otherwise, the goat can simply walk from one scoring area to the other while the trolls are battling on the other end of the field.
There are a number of technical problems in there as well:
The AI wins immediately if it scores with the goat. (Make that count one point, not three.)
The AI's trolls just move to the middle of the field where they remain for the rest of the game unless they are attacked. (Add a looping trigger with effect 'move to unit' and have them always go for the goat.)
Under AI control the goat always goes for the same spot. It's impossible for the AI to score twice this way. (Add a second trigger and make it go for the other scoring area once the first point has been scored. You can use quest variables to keep track of that.)
The militia can walk right through the walls. (Use the 'place wall' tool in the editor to create walls without holes.)
The militia sometimes attacks the players and even the dwarfs on the 'scoreboard', thereby erasing the score. (Don't use Gaia as owner for the crowd, create a third player which is neutral to the other two.)
The game doesn’t really end after the victory message. (Add a 'you win' effect after the cinematics.)
Both players being mirror images of each other would make for a perfect balance in a multiplayer scenario. However, since the AI doesn't know how to play it's almost impossible to lose here once the enemy Arcus is out of the way (see playability). Also, winning by killing the obelisks is much too easy. I think you shouldn't allow more than one point being scored like that.
That's a definite five! The idea is great, another discipline for the AoM Olympics!
Map Design: 4
The map looks good and it serves its purpose. There's still room for improvement however – a 'real' stadium/coliseum would look great, flags could be used to keep the score... But I guess this would be sort of an overkill.
There's no story, but the scenario doesn't really need one. The instructions are clear enough for this simple game. Even though you're obviously a city kid if you can't tell a sheep from a goat...
Get rid of the bugs I mentioned and make this a multiplayer scenario! This one has definitely 4+ potential.