Oh wow, this has to be my least favorite issue of Avengers yet. I can kind of see what Stan was trying to do here, but.... it's just not very well executed.
Immortus is introduced as a mysterious character with his own agenda, and it's rather unclear whether he's truly evil or not, but none of this is ever followed up on. I'm not quite sure when Immortus next appears, but it's really far from now, the mystery of his agenda and true identity is never really revealed, and the entire character ends up being rewritten completely, rendering pretty much everything he says in this story a lie. I can definitely imagine kids feeling cheated back then when remembering this story some years after it's original publication, and realizing nothing ever came of the stuff introduced here, even though it's clearly setting up for a sequel.

Immortus luring Rick to his lair with a comic book ad is dopey enough to work, but Cap just accepting the fact that the Avengers had Rick imprisoned by some supernatural wizard like this just to keep him from whining about membership is hard to swallow. Immortus' "tournament" is interesting in concept, but the battles aren't very long or interesting, and the final trial is... Cap battling some guys with pointy sticks off-panel? What?

The Masters of Evil going from having the advantage to being completely defeated just by Cap showing up is a bit hard to believe, the Enchantress just turning back time like that to save her group really feels like a cheat, and her being able to break contact with Immortus to conclude the story directly contradict what was stated earlier.

Really didn't like this one at all.

Was Cap already a "supersoldier" in the Golden Age


Yes. His origin was established in the very first Captain America story ever published, and has gone relatively unchanged since then, some minor details aside.

Out yonder, under the shining vault, among men the saying goes: 'Man, be thyself!'
At home here with us, 'mid the tribe of the trolls, the saying goes: 'Troll, to thyself be-enough!'