Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† astu (asty, Vitr. 8, 3; 7 praef.), n., indecl., = ἄστυ, a city, esp. Athens (as urbs κατ’ ἐξοχήν for Rome): omnes qui arcem astuque accolunt cives, Att. ap. Non. p. 4, 330: An in astu venit? Ter. Eun. 5, 5, 17: demigrare ex agris et in astu, Cic. Leg. 2, 2, 5; Nep. Them. 4, 1; id. Alcib. 6, 4.