EDIT: It was Madö King Granzört! Thanks to everybody that upvoted and contributed making the post active.

Hi! I've been trying to find this old anime that I used to watch as a kid. It was probably in the early 90s, but in could be from the late 80s.

What I vaguely remember is that, in this anime, the kids would enter what appeared to be some sort of dark room or dark space, and there they would be standing on top of a pentacle (like in the attached picture) and the moves they would do there would be forwarded to some monster or robot, so they would fight other monsters or robots.

I'm not sure if this qualifies as "mecha anime". I don't remember enough to be sure, so perhaps it didn't fit the category after all.

Any hint would be much appreciated. Thanks!

Magic Knight Rayearth? Or maybe G Gundam? Or Madō King Granzört?

Sounds like you might be talking about Angelic Layer from 2001 wasn't it?

Edit: Actually, no, too far from the original description, I was just basing it on the kids having their moves transferred to robots that would do the actual fighting, and it was sort of a tournament. But even if you might've misremembered some parts, Angelic Layer still seems like a big stretch.

Could it be Matchless Raijin oh?
Although I’m not sure on the pentacle

Thanks! It was Madō King Granzört!

This was mostly the image I still had in my memories after a third of a century!

Again, thank you everybody.