Merlin is Uther's ward and is locked in a tower
I'm trying to find a specific fanfiction that I read about a year ago. I think it was on AO3, but it may have been ff.net.
The fic starts when they are children. Merlin is Uther's ward, but only because Uther wants Merlin's magic under his control. Merlin spends most of his time locked in a tower, but I think Arthur and Morgana visit him.
I'm pretty sure he has to keep going through 'treatments' that Uther orders to try and rid him of his magic, but the treatments are more like torture.
I never finished it, and can't remember if it was gen or merthur.
It was quite long if I remember correctly, and was well written.
It's not The Dragon Lords Son or any of the 3 'The Kings Wards' stories that come up on ff.net
EDIT: It's Canary in a Cage by CaffeinatedFlumadiddle, which has unfortunately been deleted. But I found a link to a google drive with all of the author's fics downloaded!