After the discovery of Dirac electrons in condensed matter physics, more specifically in graphene and its derivatives, their potentialities in the fields of plasmonics and photonics have been readily recognized, leading to a plethora of applications in active and tunable optical devices. Massless Dirac carriers have been further found in three-dimensional topological insulators. These exotic quantum systems have an insulating gap in the bulk and intrinsic Dirac metallic states at any surface, sustaining not only single-particle excitations but also plasmonic collective modes. In this paper we will review the plasmon excitations in different microstructures patterned on Bi2Se3 topological insulator thin films as measured by terahertz spectroscopy. We discuss the dependence of the plasmon absorption versus the microstructure shape, wavevector, and magnetic field. Finally we will discuss the topological protection of both the Dirac single-particle and plasmon excitations.