User blog comment:San&Ashitaka/To be or not to be?/@comment-93.209.48.123-20140618073951/@comment-25358468-20140621224206

This wikia refers to them as Wall Titans but generally, among the fandom and other sources, they're called "Hidden Titans."

"Illusion" means "a thing that is or is likely to be wrongly perceived or interpreted by the senses." I think that explains it perfectly; the Survey Corps obviously, at first glance, took it to be just another part of the walls and were unaware that there are actually Titans inside them. I'm not sure what "dream" could refer to, but there can't be two meanings so, it is probably something lost in translation (much like how Shingeki no Kyojin somehow turns into Attack on Titan.)

Also, comparing an author's works with what he himself enjoys (or grew up on) is a horrible comparison... George RR Martin grew up on comic books, the classics like Superman, Spiderman and Batman, and now he writes High Fantasy. Most of his characters are lords, knights or archers, none of which were ever contained in the classic comics he read growing up.

There has never been a mention of any kind of supernatural portal in all of the series. That is literally all speculation. 90% of the series is shrouded in uncertain mystery, so assuming a "portal" could be where the Beast Titan is headed based off vague, inside exchanges between characters who we basically have no idea about (past, present, future even) is simply made up.