Or you can go down the roguelike path and generate the levels procedurally. This approach is incredibly powerful and gives traditional roguelikes a ton of replay value while being very easy to build compared to traditional static assets. Plus there are subreddits [1] [2], wikis [3], and more dedicated to the topic!
[1] https://www.reddit.com/r/roguelikedev/
[2] https://www.reddit.com/r/proceduralgeneration/
[3] http://roguebasin.com/index.php/Articles