I usually send a general and some heavy cavalry to Timbuktu. I hire some mercs when I get there and I take that city. I then build a merchant building and a town hall. (town hall prevents your merchant from being crooked) Then I build a fort on the gold and start pumping merchants into that fort. After awhile the income from gold in Timbuktu will surpass all other incomes and it will be very steady.

Also I build lots of priests there and they become my Christian hit squad and they eventually move across North Africa to Egypt then up to Turkey converting as they go.