。。
Somehow hold them off there. Do not let them enter the city.
Literal
Somehow there [at-で] support [imperative-よ]. Entry-into-city [object-を] permit [negative-imperative-な].
A pair of clipped battlefield orders in the stiff, archaic register of 時代劇 (period drama). 支えよ is the classical/literary imperative of 支える ('to support, hold up, withstand') — the ~よ imperative ending is archaic; the modern equivalent would be 支えろ. 許すな ('do not permit') is the standard bare negative imperative. 入市 ('entering a city') is a formal Sino-Japanese compound you'd mostly see in military-historical writing. Together the two clauses read as a commander barking: hold the line, and do not let them into the city.