By my estimation, you've just saved Overseer Martin, and are here, at this gate:
I like to Blink up to the air duct next to the gate. You have to jump and blink at the same time:
From there, you overlook an open area with three overseers. My preferred strategy is to Blink onto the tops of the street lights:
Blinking onto the next couple of lights gets you close enough to Blink to the stone ledge:
and from there across to the ledge of the building High Overseer Campbell is in:
You now overlook the main courtyard of the Overseer's building. Staying on this ledge will lead you to an open window where you can enter without confronting any enemies.
Ta-Da! You're in! Now to navigate the interior of the building.
Even if you haven't spent runes on upgrading Blink, I think this method will still work. Blink is probably the best skill Corvo has since it allows you to take so many new paths.