Castlevania: Circle of the Moon is a great example of how making a metroidvania is a lot more difficult than it seems. CotM should be looked at as a guide of how NOT to make one:

- All of the powerups besides the first two do absolutely nothing outside of opening new passages
- Combat stays mostly identical for 90% of the game
- Double tapping to move is really annoying
- DSS is novel, but completely luck-reliant
- Every DSS combo is either borderline useless or OP
- The best sub weapon (the boomerang) has only two locations where you can get it
- Level-up system essentially forces grinding
- No shops whatsoever, so all of the duplicate items in your inventory have no purpose
- Many of the enemies are the same knight guy with a different color
- Fast-travel points are really poorly placed, making backtracking often boring
- While not an issue in the early game, late-game bosses have irritating run-ups
- Rooms blend together really easily and most of them are just corridors with enemies
- All of the bosses are boring, with three attacks and way too much HP
- The final boss is absolute agony

In short, don't play this game. Please.