Tungsten trioxide is a binary oxide that has potential applications in electrochromic windows, gas sensors, photo-catalysts, and superconductivity. Here, we analyze the crystal structure of atomically flat epitaxial layers on YAlO3 single crystal substrates and perform nanoscale investigations of the ferroelastic twins revealing a hierarchical structure at multiple length scales. We have found that the finest stripe ferroelastic twin walls along pseudocubic < 100 > axes are associated with cooperative mosaic rotations of the monoclinic films and the larger stripe domains along pseudocubic < 110 > axes are created to reduce the misfit strain through a commensurate matching of an effective in-plane lattice parameter between film and substrate. The typical widths of the two fine and larger stripe domains increase with film thickness following a power law with scaling exponents of similar to 0.6 and similar to 0.4, respectively. We have also found that the twin structure can be readily influenced by illumination with an electron beam or a tip-based mechanical compression. (C) 2015 AIP Publishing LLC.