User blog comment:Dominic1743/“The Trap” Discussion/@comment-79.115.70.229-20200120113459/@comment-108.36.76.116-20200122094211

Castiel did survive on his own in Purgatory before. He's also grown in wisdom and wiles, and has learned things from Dean on being ruthless.

If a bodyless monster soul (shapeshifter in form of Castiel)  tried to cross the portal, I think it would become a monster ghost. Castiel and Dean both went in with bodies.