In wireless and mobile networking, volatile environ- mental conditions are a permanent challenge, resulting in a demand for cross-layer optimizations. To further increase flexibility, we believe cross-layer architectures should adapt themselves to these changing conditions, just as they adapt the network stack, devices, and applications. In this paper, we propose CRAWLER, a novel cross-layer architecture that combines three core properties: signaling between all layers and system components; extensibility and adaptability at runtime; and high usability for cross-layer developers. CRAWLER increases flexibility, as well as expediting and simplifying cross- layer development.