This scenario takes place roughly as the same time as Reconquista, further inside the New World.
The British have brought down all of the Circle's fortresses in the Americas, and the fort you're in is perhaps the last one in the whole continent.
The British outnumber you heavily from the beginning, and receive reinforcements from time to time, so a head-on engagement is out of the question.
You must get garrison Commandant Schreiber and his men to the safe zone down south. The Circle will send whatever forces it can still scrounge up once this is done. Further reinforcements will arrive once a certain point in the operation has been reached.
The surrounding forests are filled with treasures. Getting as many as possible will help you get an edge over the British.
Also, there are many tribes that call these plains home. Allying with them will help in the long run.
Copy or move the survival.age3Xscn file to your Scenario folder (usually C:\Users\Your User Name\Documents\My Games\Age of Empires 3\Scenario) and the Seville_HC.xml file to your TWC Campaign folder (usually C:\Program Files\Microsoft Games\Age of Empires 3\Campaign\War Chief).