
The BP will only be for a GB that is associated with the age of the building you have hit. With the exception (currently) of Observatory 1, Temple of Relics 2, and Oracle of Delphi 3 you have the chance to earn a BP from any building that you aid/plunder. Depending on how badly you need BPs will determine what method, or combination of methods, you’ll want to employ to get them. There are a few ways to go about this, with some methods being more effective than others. The biggest struggle when it comes to this though, is acquiring all the blueprints you need in order to plant a GB in your city.

Hands down, no questions asked, Great Buildings will effectively change and skew your city to the play style you desire (especially for GvG, but GE too).
