Castillo, the 18-year-old star from Colombia, delivered a nice cross to set up Brek Shea's goal in the 29th minute, then scored his second goal of the season in the 43rd minute.
Although Philadelphia attempted nine shots, FC Dallas goalkeeper Kevin Hartman had to make just one save to record his third consecutive shutout, but it was a tough one, as was forced to dive to stop Danny Mwanga's 30-yard blast in the 77th minute.
Dallas improved to 5-3-2 and ran its unbeaten streak to five (4-0-1), a span in which it was outscored opponents 7-2. It also coincides with the broken ankle sustained by 2010 MLS MVP David Ferreira.
The Union (4-3-2), who entered the match having conceded just four goals all season and just one from the regular flow of play, surrendered both goals that way in this one and are winless in three games (0-2-1).
Dallas opened the scoring when Castillo burst through two defenders and into the penalty area and slid a perfect pass through the goalie box to Shea, who slipped it past goalkeeper Faryd Mondragon. For Shea, who signed a contract extension through 2015 on Thursday, it was his third goal of the season.
Jackson Goncalves set up Dallas' second goal with a nice service from the right wing into the middle, where the 5-foot-7 Castillo leaped over 5-10 defender Brian Carroll and headed it home.
(Copyright 2011 Philadelphia Union. Reprinted by permission)