Oh, there is no 'wall'. If you want to scope your missives, change the scope of a new message (not a reply… replies are scoped to the original message) below the area where you enter in your message (it'll be set to something like "people in your circles" or "public" or somesuch). You can add multiple values, including selecting people individually. Note that your prior scope is the default when making a new message, (it's sticky until you change it again) so be sure to double check it before you share it.
Also note that G+ encourages longer posts, and long reply threads… don't be afraid of them ;-)