If you mean the Cavalier desktop case from Coolermaster, the only way Coolermaster could make it slim was by using a non-standard (shorter) PSU.
I don't think I like that compromise at all. That non-standard PSU ran hot and it was noisy, and worst of all, you can't change out the PSU, which was why I got rid of my Cavalier after just 1 month.
Silverstone does have a low-profile MicroATX case though... The LC11. As with most MicroATX cases it does have a limited number of drive bays though.
I also got another HTPC using the Silverstone LC02. That one takes a Full ATX motherboard (!) in a package that is indeed under 12cm, but you are limited in the number of expansion slots because you need to use risers. And of course the PSU is also non-standard in this case, because of the limited amount of room available... Some may not find the non-standard PSU in the LC02 quiet enough or cool enough either, unfortunately.
My LC03 in its environs
The LC03 (closeup)
People who visit me usually couldn't tell if that LC03 was a computer, since it sits behind me (I'm using a wireless keyboard and mouse).
HTH.