The Dripstone Wall

The garden built by Albrecht of Wallenstein was divided into several major parts. Part whose most important element is the artificial rock or dripstone wall was called the secret or secluded garden (giardino secreto or giardino segreto). This section created a secretive and mysterious area where elements of artificial and real living nature mingled and contrasted, where the feeling of uncertainty was heightened by the startling images of frogs, snakes, monsters, grotesquely formed faces and various kinds of animals hidden in the recesses of dripstone wall. The illusion of mystery and uncertainty is further enhanced by numerous illusionary hints suggesting the wall might be penetrable by secret corridors or passageways. 

The artificial rock fits in naturally with the aviary, which also lends an important acoustic feature to the garden. All these the joining of real living and artificial nature, the juxtaposition of contrasts, the creation of illusion and the placement of bizarre details-are stylistic and creative principles of the mannerist or early baroque garden, conceived on the model of Italian gardens.

Closer investigation reveals the wall is made from a haunting, uncanny assemblage of stalactite-like rocks. Signs along the wall note that, if one stares hard enough, it's possible to make out human and animal faces peering out from within the wall's recesses.

Adding further fuel to the site's disconcerting nature are other strategically placed clues that the Dripstone Wall itself could, perhaps, contain secret passageways wending through its interior, accessible only by those bright enough to discern the pattern of access. Throughout nearly 500 years of years of existence, no one seems to have found a way in, but, thankfully, that hasn't yet stopped visitors from dreaming about solving the mystery.

Sources:

http://www.atlasobscura.com/places/dripstone-wall-2

Pic Source:

https://www.flickr.com/photos/63042497@N00/8014647023/