Above: This arch was created in the unusual manner of leaning two curved blocks together. This method will work only if the blocks do not slip outward from the pressure of leaning together. This arch requires some understanding of two counteracting pressures.
Left: Children see an arch as an invitation to walk through the vacant space.
Right: Making a doorway using a lintel (the top block that lies across the two vertical blocks) requires some thought about spacing and support. As you see here, arches contain enough complexity to elicit cooperative play.
what it means
It is no small problem to figure out how to make an enclosed vacant space in a vertical structure. The window, door, or arch is bounded on all sides yet remains vacant in the middle. It is much easier to make a stack or a solid wall. These openings can become a focus of play, such as the port hole on a ship or a space were the inside person can talk to the outside person without letting the outside person in (remember the Emerald City gate keeper in The Wizard of Oz?). Starting from something as simple as the arch, the children might create extensions of the arch or think that the play space also needs an escape archway in the back. Doorways provide children with an implicit instruction on how to navigate and use space.
2-4 year olds
Very young children may find crevices and spaces to enter but will not make a well-formed arch or window on their own. Nevertheless, body-sized empty spaces to a young child have the same appeal as a slightly opened cabinet to a cat. They will want to know, “what is in there, can I go through, does entering the space make my parents smile or call my name?” They explore what can be navigated in both expected and unexpected paths.
4-7 year olds
The older children will become interested in the engineering of these enclosed yet vacant spaces. They will go through trial and error to get a hole where they want it. Here you will see the advantage of the foam blocks that can be toppled without consequence when children want to destroy and try again.
7-10 year olds
Still older children will move from the sheer engineering of these spaces to decisions about how many spaces, where, and for what purpose. A castle wall may have both a window and turrets (gaps with no lintels as in the second graphic above) and these architectural differences will have to be negotiated and explained in terms of their purpose.