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Architecture captures the vision
In an IT environment the architecture can evolve incrementally but (and it's a BIG BUT), you will always find that there is a core architectural footprint from which everything else gravitates. There is a core set of functional components that represent the architecture and even
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though the architecture evolves those components remain consistently present throughout the evolution of the architecture. You will see such examples as 3-tier architectures, master-slave architectures, dual-redundancy systems etc. In the same way that software patterns can be categorised as either structural, behavioural, creational etc, as defined by GOF, these same patterns and more exist at an architecture level. Also, architecture is more than just patterns, it has to consider security, costs, hardware, people, usage, scaleability, maintainability, testability, how do you protect the investment, means of distribution, vendor selection, development approach and the list goes on...
So can you really do the above in a TDD manner? I think not. Incrementally yes to some degree, TDD no, no, no!!! Software is only one part of the architecture stack.