The Architect

ARCHITECT

The architect appears at main castles in all worlds, but can be used even in outposts. It lets you unlock the ability to upgrade buildings to new levels. Once the architect is no longer there, you cannot upgrade the new buildings, the architect is now needed to level up some of the legendary level buildings too when you get to higher levels.

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To use the Architect's special ability, you need a building of the appropriate level, and then click on your resource building and click the normal upgrade button. The button is grayed out when there is no architect, like this: http://prntscr.com/74a26r.