"Once you're blind, you have to set new goals," Castro said.
He set them higher.
Not content with just staying in the Army, he is the only blind officer serving in the Special Forces - the small, elite units famed for dropping behind enemy lines on combat missions.
As executive officer of the 7th Special Forces Group's headquarters company in Fort Bragg, Castro's duties don't directly involve combat, though they do have him taking part in just about everything that leads up to it.
"I am going to push the limits," the 40-year-old said. "I don't want to go to Fort Bragg and show up and sit in an office. I want to work every day and have a mission."
Since the war began in Iraq, more than 100 troops have been blinded and 247 others have lost sight in one eye. Only two other blind officers serve in the active-duty Army: one a captain studying to be an instructor at West Point, the other an instructor at the Combined Arms Center at Fort Leavenworth, Kan.
Castro's unit commander said his is no charity assignment. Rather it draws on his experience as a Special Forces team member and platoon leader with the 82nd Airborne Division.
"The only reason that anyone serves with 7th Special Forces Group is if they have real talents," said Col. Sean Mulholland. "We don't treat (Castro) as a public affairs or a recruiting tool."
An 18-year Army veteran, Castro was a Ranger before completing Special Forces training, the grueling yearlong course many soldiers fail to finish. He joined the Special Forces as a weapons sergeant, earned an officer's commission and moved on to the 82nd - hoping to return one day to the Special Forces as a team leader.
Then life changed on a rooftop outside Youssifiyah, Iraq, in September 2006.
Castro had relieved other paratroopers atop a house after a night of fighting. He never heard the incoming mortar round. There was just a flash of light, then darkness.
Shrapnel tore through his body, breaking his arm and shoulder and shredding the left side of his face. Two other paratroopers died.
When Castro awoke six weeks later at the National Naval Medical Center in Bethesda, Md., his right eye was gone. Doctors were unable to save his left.
The Blinded Veterans Association estimates 13 percent of all combat hospital emergency procedures in Iraq have involved eye injuries and more than half of the soldiers with traumatic brain injuries also suffer some visual impairment. That makes them the third most common injury - behind post traumatic stress disorder and brain injuries - in Iraq.
"What he is doing is a strong example that blind individuals can lead exciting and meaningful careers," said Thomas Zampieri, director of government relations for the association.
After 17 months in recovery, Castro sought a permanent assignment in the service's Special Operations Command, landing duty with the 7th Special Forces Group. He focuses on managerial tasks while honing the group's Spanish training, a useful language for a unit that deploys regularly to train South American troops.
"I want to support the guys and make sure life is easier for those guys so that they can accomplish the mission," he said.
Though not fully independent, he spent a weekend before starting his job walking around the Group area at Fort Bragg to know just where he was going. He carefully measured the steps from car to office.
"Obviously, he cannot do some things that a sighted person can do. But Ivan will find a way to get done whatever he needs to get done," Mulholland said. "What I am most impressed with, though, is his determination to continue to serve his country after all that he's been through."
Castro works out regularly at the gym and runs, his legs powerful and muscular. And though he has a prosthetic right eye and his arms are scarred by shrapnel, his outsized personality overshadows his war wounds: Nobody escapes his booming hellos, friendly banter and limitless drive.
He ran the Boston marathon this year with Adm. Eric T. Olson, commander of the U.S. Special Operations Command. Last year it was the Marine Corps Marathon. He wants to compete in the Ironman triathlon in Hawaii and graduate from the Army's officer advanced course, which teaches captains how to lead troops and plan operations.
Mulholland said Castro, who was awarded a Purple Heart like others wounded in combat, will always be part of the Special Forces family.
"I will fight for Ivan as long as Ivan wants to be in the Army," Mulholland said.
Married and the father of a 14-year-old son, Castro still needs help getting to the gym. He recently needed an escort to the front of the headquarters company formation, where he promoted a supply clerk.
Once in front, Ivan took charge.
Affixing the new soldier's rank to his uniform, Castro urged the soldier to perform two ranks higher. In the Special Forces, he said, one has to go above and beyond what is asked - advice he lives by.
"I want to be treated the same way as other officers," Castro said. "I don't want them to take pity over me or give me something I've not earned."