last line of defence between

[kingston_123]

Det. Frank enters the sex shop and asks the woman who is working there.

Frank: Listen. We're looking for Hector Savage. Where is he?
Woman: Why should I tell you?
Frank: Because I'm the last line of defence between sleaze like this and the decent people in this town.

What is the meaning of "last line of defence between "?
I heard the idiom "line of defence", but here it is used differently. So how can I understand it?

Source: The Naked Gun 2½: The Smell of Fear (1991)

 

[Charlie Bernstein]

the last line of defense =

- the final protection
- the only barrier that remains
- the one obstacle that still stands

It's figurative. It comes from military language. Armies create literal lines of defense. Figuratively, anything that blocks an assault is a line of defense.

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PS - I use an S in "defense" because I'm an American. C is correct for British English.