For lighting the other towers, you have to solve puzzles - there aren't staircases for the other lights. For instance, you need to solve the bell puzzle in the area where you talked to the king ghost. You'll need to find an inventory item elsewhere in that spirit land to do this.
For me, this spirit land was too difficult at first, as some of those creatures could kill me instantly. So instead, I went back to the castle. If you go to the leftside on the first floor, there is a staircase that is now opened to you, and on the second floor and upwards, you will encounter a lot of enemies, but most of them are easier then those in the spirit world, or at least I thought so! It's important to find some artifacts so you can get some stronger spells. Also don't forget to allocate your experience points, as you gain them rapidly in the beginning, and it makes a real difference in surviving.