Using a rotating cryostat, we have observed nucleation, annihilation and strong intrinsic pinning of continuous vortices in a slab of superfluid 3He-A containing stable textural defects. A model of a critical state set by either the critical velocity for vortex nucleation or pinning strength is developed. It predicts a hysteretic dependence of trapped vorticity on the angular velocity of rotation, in agreement with the observations. We argue that the static defects responsible for nucleation and trapping of vorticity are networks of domain walls between regions of opposite orientation of the l̂-vector. © 2005 Springer Science+Business Media, Inc.