The Colts waived Castle on Sunday, Joel A. Erickson of The Indianapolis Star reports.
Castle signed with the Colts in May as an undrafted free agent out of Duke. The running back participated in all three preseason games, accumulating 18 rushes for 88 yards. The UDFA will now be subject to waivers. If Castle goes unclaimed, he will become a free agent able to sign with any organization.