I thought he might be talking about where we host our stuff as our castles, but he means it in a much more personal and direct way - web browsers (and other internet-abled apps, I would add) are extremely personal spaces where we invite content and code from outside the walls. I think I have the right to make sure guests leave surveillance devices and weapons outside before entering.
So here is a helpful fact: we don't go anywhere when we use our browsers. Our browser homes are in our computers, laptops and mobile devices. When we "visit" a web page or site with our browsers, we actually just request its contents (using the hypertext protocol called http or https).
In no case do we consciously ask to be spied on, or abused by content we didn't ask for or expect. That's why we have every right to field-strip out anything we don't want when it arrives at our browsers' doors.
Source: The Castle Doctrine – Medium