"When I was little, about 6, my mom gave me the option to either go to summer school or swim in the morning," she said. "I was like, 'I'm not going to summer school.'"
Castleman hasn't stopped. She completes her high school career today and hopefully Saturday at the state meet at the Rec-Plex in St. Peters, Mo. It will be her fourth consecutive trip to the state meet.
"I'm excited," she said. "Freshman year, it's just something I just swam through. I wasn't trying to get a state cut. It just kind of happened. After that, because you made it freshman year, you should probably make it or you look bad."
Castleman will compete in the 100 freestyle and 50 freestyle.
"At first, I just wanted to make state," she said. "Now that I'm going to get there, I want to place higher than what I'm ranked."
Castleman's relationship with swimming is a rocky one. She said she enjoys the friends she'd made in the sport, but listed commitment to swimming as the hardest part about it.
"There are times I just want to stop," she said.
She manages her love/hate relationship with the sport by limiting her involvement. She only swims during the winter and summer.
"I sit out fall and spring," she said. "I need it because I get burned out."
Her attitude toward the sport depends on what she's doing.
"Out of the water, you love it," she said. "It's when you get into the water and actually do it, then you start to hate it."
That changes during a meet. The thrill of competing is what keeps her swimming.
"The meets are very rewarding," she said. "I enjoy meets a lot more. You're in the water for maybe, what I do, maybe four minutes the whole meet."
Her current plan is to attend the University of Missouri in the fall, but that could change. She isn't sure if she's ready to give up swimming.
"I kind of wanted to swim in college, but now that I've thought about it, it's hard to find a college that has your major and has a lot of choices because I don't know what I want to do yet," she said. "I'd have to go to a smaller school because I couldn't do Division I."
She said there's little strategy involved in the two events she'll swim at the state meet. Both freestyle events basically are sprints. She said the 100 freestyle is more difficult than the 50.
"The 100, you're tired," she said. "The last 25 your legs are numb. You just keep moving. You can't really feel anything so it doesn't really matter."
Even though it's a sprint, she likes to know where she stands, so she'll peek at her opponents during a race.
"I don't lift up my head, but I can look over and see their body," she said. But it's hard kind of hard to tell because you can't tell whose arm is going to stretch further. I always look. I don't think you're supposed to, but I do."
She hopes to be looking behind her instead of in front of her during this weekend's meet. She knows she won't have a problem getting motivated for her final high school meet.
"It's easier if you swim with better people to keep going," she said.