The C64 version was a later release than Amiga's Defender of the Crown.
Subsequently, Cinemaware improved the gameplay in various ways for the C64.
The 8-bit game is more challenging and has greater variety. It has more random events, more siege options, more battle options, etc. You can also "Pass" a turn. Unlike on the Amiga, buying an army on C64 DOTC costs you a turn. This aspect makes the C64 game more difficult to complete - thus, more exciting.
Like you, i played the C64 version first and I was bit disappointed playing the Amiga's. As far as castle sieges go, was miffed not to see the Amiga DOTC have greek fire + disease missiles!
Also, wanted to see the castle up in flames on the 16-bit!!
The C64 version is the best - on all platforms!