The Terrarium House can be purchased at level 30 for 70 Diamonds.
To unlock it, you must first complete the preceding quest "Dormouse in Distress" (obtained at level 28). Once you have done that, you'll be able to buy the Terrarium House in the Shop under Buildings > Feature.
Once built, clicking on the Terrarium House and selecting the mouse button will allow you to enter the Terrarium House.
The Terrarium House is essentially a whole zoo of its own, with a special focus on the smallest zoo critters. Your view will be zoomed in significantly so that you can enjoy this whole new world!