I know that the "momentum" technique can sometimes be buggy, but I tried my best. If the fling doesn't work, try the following:
1. Repeat until it works
2. Try jumping into the sideway portal "elegantly", do not directly walk into it
Kira, this. Is. One. Of. The. Best Game I have ever played in castle. It's so amazing how you made portal in 2D and yet still having amazing puzzles. Bravo bravo. I don't know what more to say but I still have a lot on my mind.
Thanks 903 for probably the best portal "knockoff".