That's Old North Church, in the North End of Boston, a few blocks from my son's apartment. As Wikipedia dryly sums it up:

On April 18, 1775, probably a little after 10 P.M., the 191 ft (58 m) steeple of the Church served a military purpose. Paul Revere told three Boston Patriots to hang two lanterns in the steeple... The lanterns were displayed to send a warning to Charlestown Patriots across the Charles River about the movements of the British Army... The signal only lasted for a few brief moments to avoid catching the eyes of the British troops occupying Boston, but this was long enough for the message to be received in Charlestown. They had kept someone looking at the steeple all night.

Or, as Longfellow had it:

He said to his friend, "If the British march
By land or sea from the town to-night,
Hang a lantern aloft in the belfry arch
Of the North Church tower as a signal light,--
One if by land, and two if by sea;
And I on the opposite shore will be,
Ready to ride and spread the alarm
Through every Middlesex village and farm,
For the country folk to be up and to arm."

The tower still stands tall,
and I think the need remains,
when liberty's under assault,
to take a horse by the reins,
and ride with a cry far and wide.