A phase with composition NaNb7O18 was first reported by Shafer and Roy in 1959 and then confirmed by Appendino in 1973. Several other contemporary studies of the Na2O-Nb2O5 phase diagram missed this phase. The ambiguity likely stems from the fact that, at ambient temperature and pressure, NaNb7O18 is a metastable phase; its phase stability was reported to be above 980 C up to its incongruent melting point of 1265 C. Although the studies by Shafer and Roy and by Appendino reported diffraction data, the first structure model for NaNb7O18 was given by Marinder and Sundberg from electron and powder X-ray diffraction in 1984. The covalent metal oxide framework of NaNb7O18 has a Wadsley-Roth-like structure comprising (4  4 -2) blocks of corner-sharing octahedra. Relative to the conventional Wadsley-Roth structure type, two corner octahedra have been replaced by square-planar sodium cations, analogous to the (5  3 -2) blocks in NaNb13O33. Owing to limitations in the data quality, only the square-planar Na1 site in the large "double" tunnel of the [Nb7O18] - framework could be convincingly located, but that gives a crystal structure formula of Na0.5Nb7O18 and is only half the sodium required for charge balance. Given the white color of the compound, the possibility of even a small concentration of reduced niobium can be excluded, so it was suggested that the remaining Na + should reside in perovskite-A-site-like environments within the blocks. In the context of ionic transport down the parallel square-cross-section tunnels in Wadsley-Roth compounds, Na + in these A-sites would be expected to be conductivity-blocking point defects. There is some precedence for these defects in the related compounds NaNb13O33 and AgNb7O18. The stoichiometry of NaNb13O33 is commensurate with a fully ordered structure with exclusively square-planar sodium and this was the structure model of the compound for many decades. Very recently, sodium-ions in perovskite-A-site environments have been identified in NaNb13O33 by solid-state NMR spectroscopy and suggest a crystal chemical formula of [Na sq.pl. ]0.9[Na cuboct. ]0.1Nb13O33 with about 1.5% occupancy for each A-site. Thanks to the high X-ray and electron scattering factors of silver, Ag + has been detected on the A-site in AgNb7O18 from powder diffraction and transmission electron microscopy. Though not the focus here, it is also worth noting that, beyond these point defects, Na + conductivity-blocking line defects have also been observed by careful electron microscopy in NaNb13O33 and NaNb7O18. NaNb7O18 thus serves as an excellent system to demonstrate the extreme defect tolerance of the Wadsley-Roth block structure to tunnel-blocking defects. We first quantitatively characterize the atomic structure of NaNb7O18, particularly the sodium positions and occupancies, via neutron diffraction and NMR crystallography. Then, we show that high-rate Li + intercalation is still possible with ~16% Li + -pathway-blocking point defects and micrometer-scale particles. Finally, we use operando synchrotron diffraction to characterize the response of the niobate framework to lithium insertion, revealing two discontinuous phase transitions and reversible niobium metal-metal bond formation. Most Wadsley-Roth structures undergo solid-solution lithiation, which was believed to be a source of their high-rate performance. Metal-metal bonding in Wadsley-Roth structures has recently been proposed on the basis of computational investigations of the lithiation of Nb2O5 and TiNb2O7 but experimental verification is sparse, until now. Overall, these results suggest that neither a high defect concentration nor two-phase reactions are necessarily detrimental to high power and fast charging. This demonstration of defect tolerance opens new avenues for experimental and computational materials design with respect to both structure and composition spaces for applications involving ion conduction.