Good Practice to Castle Drop on Ally base?

Who knows if your ally may not be as capable as you are?

They may need to be safe from enemy attacks, but their economy and army may not be well-developed yet hence they need the Castle for their security.

If you have stone for one castle, do you build it for yourself, or for your ally?

If you are pocket, better to build a castle on flank. But there are many nuances.

You can also build a castle at home as a pocket if you need UU or if you are raided by opponent.

Also, there is no point of building a castle to protect your team mate weaker than you because he will not be able to defend that castle if he plays bad.

I mean difficult to answer your question, it’s all about good decision making in a specific situation