build walls around

何でもない表現ですが、記事が面白かったので扱います。
What are Instagram and Twitter fighting about, and why? - The Economist explains

The competition between the two services is good in some ways, because it is forcing them to add new features and improve their offerings. But this benefit is outweighed by the drawback that Instagram and Twitter are becoming steadily more closed and restrictive. The fight between them is merely the latest illustration of the fact that when it comes to online social networks, the interests of the companies that run them do not always align (and may well conflict) with the interests of users. For services that wish to make money from advertising, it makes sense to be open initially ― but then, having attracted a large base of users, to start building walls around them.

build walls aroundは「城壁を造る」という意味ですが、ここでは「固持する」「万全を以って守り始めた」「守りに入った」などの意味で使われています。よく歌詞にもbuild a wall around my heartなどが登場します。何やら対象を閉じ込めて外に漏らさないようにするという印象を受けます。こういった表現は、自分では思いつきません。悔しくて挙げてしまいました。(Othello)