Synonyms for Fastness (same or very similar meaning)
WordNet sense 1 (the quality of being fixed in place as by some firm attachment):
fixity, fixture, fixedness, securenessWordNet sense 2 (a strongly fortified defensive structure):
strongholdWordNet sense 3 (a rate (usually rapid) at which something happens):
speed, swiftnessFrom the ODE community, based on WordNetadd/edit