Preheat oven to 350°F (175°C). Use a deep-sided 10-inch pan or wrap the outside of a 10-inch springform pan with aluminum foil to prevent leaking. Sift together flour, baking powder, salt, and cinnamon, then set aside.

In a saucepan over medium heat, combine brown sugar and 1/2 cup butter. Bring to a boil, then pour into the bottom of the springform pan. Sprinkle with sliced peaches.

In a large bowl, cream together 2/3 cup margarine and white sugar until light and fluffy. Beat in the eggs one at a time, then stir in vanilla. Alternately, beat in the flour mixture with the milk. Pour batter over caramel and fruit in the pan.

Bake in the preheated oven for 90 minutes or until a toothpick inserted into the center of the cake comes out clean. Let cool in the pan for 10 minutes.