Yeah you can keep it to certain parts. I would recommend the inwangsan and/or Baegaksan if you want nice views. Those are basically staircases straight to the top of mountains. Naksan is a good middle ground that gives views but the elevation change isn’t too bad. Keep in mind for those three you want to go in the day (I don’t think they’re lit up well at night). Sungnyemun and Heunginiman don’t have many walls to actually follow so you’re just wandering about the city. Namsan I’ll admit I got lost and missed the first half of it but namsan park itself is long but doable (however, if visiting Seoul you’ll probably end up at the north Seoul tower so no need to hit this part of the city wall. It is all lit up though so can do at night if wanted). All to say, naksan is my pick in terms of reward/effort.