One of the achievements in Triple Town is to build a treasure chest in a regular game. I have played a lot of Triple Town and never seen a treasure chest. How do I create one?
There are two ways to build a treasure chest.
One is to match three cathedrals. Cathedrals are created by matching three churches which are in turn created by matching three gravestones which are created by killing bears (either by trapping them or with the robot).
The other method is to match three rocks. There's usually one or two rocks in your starting position, more rocks can be created by putting a crystal down so that it will not match two other objects. Once three rocks match they form a boulder which can be broken apart with the robot to find a treasure chest inside.
Three boulders can be matched to create an even larger treasure chest, however, I'm not sure if there's an achievement for it.