Both! The gameplay more specifically, but I'm always interested in art as well ^^
That's such a wide and complex topic, So unfortunately I can't give a detailed explanation about it all in 1 reply.
For gameplay I find experimenting a lot to be very helpful, especially with the basic variables(enemy hp, run speed, bullet size, bullet speed, frame data, fire rate and so on ). Making the game fun on a basic level(like just running around and attacking enemies) is super important. The camera should also be carefully considered, like it's speed, zoom level and behavior.
In twwwr I spent a lot time just iterating on the most basic elements, with the game just having a player and 1 enemy. Instead of giving the player a complex moveset I only gave them one attack and made sure it was as fun as possible. After that the design was mostly coming up with bullet-hell patterns.
Game feel is also a big deal, and I learned a lot about it from here and here, as well as just looking at other games.
Hope this helps.
That wasn't too long at all! Thank you so much fax! It's ironic that I was thinking about trying to mix those styles lol resident evil and silent hill are some of my biggest inspirations as far as horror goes and ofc that's the genre that i want to work in lmao. Thanks a bunch that really helped ^^!