As Austria, acquire a City-State with 15 or more units through Diplomatic Marriage.
Requirements[]
Gods and Kings
Notes[]
(Prior to November 1, 2012, this was "As Austria, replace another civ as allies with a City-State and acquire the City-State through diplomatic marriage in the same turn.")
This achievement will not trigger if you have One City Challenge enabled because any cities acquired in this mode will be immediately razed (including capitals).
Strategy[]
The easiest way to do this is to ally with a city state, save up enough gold to have a diplomatic marriage, then fill all of their tiles with your combat units. The total of your units and theirs should be 15 or more. Gift all the units at once (using the unit's gift command, not the "gift a unit" option from the City State pop-up). If you do not complete the diplomatic marriage in the same turn, some of the units will be disbanded.