The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

I'd call it a
"Fortress of Solitude for babies."
The Big Bang Theory Season 10, Episode 6

...all my other SOLITUDE.
The Big Bang Theory Season 8, Episode 7

Welcome to my fortress of SOLITUDE.
The Big Bang Theory Season 8, Episode 7

...I will be enjoying a blissful evening
in my personal Fortress of Solitude.
The Big Bang Theory Season 2, Episode 21

in the SOLITUDE of a cold cell.
Game of Thrones Season 7, Episode 6