Farming on any other type of land is not permitted. The one resource that has changed the most is gold. It s no longer simply produced by mining. It is now also a product of your economy as a whole. The economic model in this game has undergone a radical restructuring.
Trade is now just as important a factor as military strength in determining who rules the world.
In AoE all you could do with money was buy units and buildings. Now you can use it indirectly against your enemy's economy. There is in Age of Kings a shared global market, with fluctuating prices and supply and demand. Let's say your enemies, the evil Franks, have nearly deforested their area. If you have any extra cash, use it to buy up the global supply of lumber.