I'm happy to say that the nostalgia comes through nearly unspoiled. The graphics remind me of the Amiga version whose screenshots I'd envied back when I played the Commodore 64 version until the disk drive wore slam out. The music is more modern, and I think I actually prefer the SID version.
As for gameplay, it's exactly the same. You play a little RISK-like strategery, claiming land and building an army. Occasionally you raid castles, try to rescue a damsel, or joust at a tournament. (Ok, there is something new in the tournament. I don't recall ever bashing my opponent in a one-on-one mace battle.) Ultimately, you want to take over the entire country and claim the crown.
The random setbacks (losing a territory, money, or sabotaged catapults) feel a little too frequent for my tastes. And the swordfighting I found to be frustratingly hard. In the original there were only two opponents, now there are ... four? Forget it, I'll stay a bachelor until I'm king. Also, how the heck does jousting work? It seems completely random to me.
I've seen some reviews complain that this game could have used some extras. Ok, sure, paying full price I can understand the disappointment. On the other hand, it makes a perfect cheap game that is way easier to pull out and play than my Commodore 128 system.
(Or, you could just play the official versions online. For essentially free.)
Is the GBA version essentially the NES version or the Amiga? I was (and still am to some extent) a Defender of the Crown fanatic and there were several variations. The Amiga looked the best but I liked the NES gameplay the best. It had the mace fight after the jousting and you used the crossbow to defend when one of your castles were attacked
I didn't recall the mace battle in the NES version, but I can check later.
This version does not have a crossbow minigame.
Except for (a) five (not four as I suspected) guards when rescuing a damsel and (b) mace battles, this appears to play like my memory of the Commodore 64 version.
Not sure if that helps. I'd say pick it up cheap, especially if you're really a fan.