Here is the sentence of my question.
希望に満ち溢れていたはずの"あなた"がなぜあんなに荒凉とした場所にうずくまっていたの。
I can understand what it's trying to say to some degree yet it confuses me why とした is used in the sentence.
With the wonder I checked とした up in the dictionary and got no explanatory answer. And I wonder if it's an adjective or a particle added to modify nouns and turn them into adjectives.
> the と is not optional with these タリ活用の形容動詞
Ah, that makes sense! I missed that. Then, as you said, タリ活用の形容動詞+と + した might require other answer than the one of 副詞 (such as ゆっくり) + と + した. But still I've found the one of タリ活用の形容動詞+と + した and this Kismet's question is duplicate. If it's not enough explanation to you, I think you should post your own question focusing on the difference between タリ活用の形容動詞+と+した and 副詞 +と+した or other your concern.