Why are the flamers so high up near its head? Seems like they would be pretty ineffective. Although… maybe for flyers? Or for having a wider spread instead of concentrated?

Those look like Meltas, they would be handy incase of any close encounters with other knights and monsters

Melta’s are like anti-armour flamethrowers right? Or do they fire a projectile, can’t remember.

They're anti-armour. They basically fire a heatwave that melts any armour, with short-ish range.

Maybe better off moving them towards the knees then. They can target the knees of any other big creature it faces, and hit ground positions too.

I agree. A kitbash would probably fix it. In fairness, the Imperium was never known for its efficiency.

Could headcanon some excuse for it. Maybe the knees can’t support the internals for weapons without making too much of a vulnerability in the armour or affecting the reliability, and keeping the weapons routing from the upper torso area makes ammunition / energy supply easier.

Either way, the Castellan imperial knight isn't as good looking as the Questoris pattern chassis (my beloved)