Spellcaster is an action-platforming game with adventure and RPG elements. The core of the gameplay centers around the action-platforming segments where the hero, Kane, must jump and shoot his way through various enemies and hazards. These sections sometimes end in bosses and Kane's basic jump and shoot functions are supplemented by special spells that can allow to do various things like fly, heal, or fire a powered-up shot. On the other hand, the story is told in a very different fashion, closer to adventure games where you choose different action options, like moving, talking, or looking around. The combination of these two very different styles is interesting at first...however, there are issues with both.
The issue with the adventure / story segments is primarily that the story is a bit convoluted and doesn't really draw you in or provide you with a great reason to keep playing. There are also segments where it is difficult to know what you should do next (so keep a guide handy!). The core action-platforming segments suffer since Kane has sluggish movement coupled with dreaded knock-back on hit. This generally makes playing some segments painful. On the other hand, good / clever use of spells can sometimes trivialize what was likely meant to be challenging segments so you can either break the game or be frustrated by it. By the way, those spells require spellpower that needs to be grinded for, so if you want to go the "easy" route, you'll have to dump some time into mindless grinding.
While I did gripe about this game a lot, it doesn't mean the game is all bad. It is a unique experience on the Master System. Also, once you get used to the controls of the action-platforming segments and some of the mechanics, it doesn't become such a chore to go through those pieces of the game. While I wouldn't say the story is especially enthralling, the the story is a major part of the game, which isn't very common with many 8-bit games. All-in-all, I'd say most people can probably find better retro experiences. On the Master System, if you are looking for storyline, probably try Phantasy Star. If you are looking for good platforming, there are a ton of games (Alex Kidd in Miracle World, Wonder Boy series, Sonic series, Illusion series, Asterix, etc.) that offer so much better than this. However, nothing really offers a blend like Spellcaster and if you have already tried everything else, this isn't something so bad it should be skipped.