In The Heart Of

The "heart" of something is its center, its core, its most vital area. This can be figurative, or in relation to territory, it can be geographic in nature. Ex.: "Protesters set fire to at least three police cars in different parts of the city, including one in the heart of the financial district." (Subject: G20 riots) This means, in the center/ the core of the financial district (of Toronto, in this case).