Abstact: Worldsheet theory of strings is a generic Weyl-invariant 2d gravity and is customarily  quantized by requiring vanishing of Virasoro constraints on physical string states up to spurious states. This customary choice obscures WQEP, Equivalence Principle at the Quantized Worldsheet theory level. We discuss that QWEP for closed strings can be made explicit by requiring Sandwich Virasoro Constraints (SVC), requiring vanishing of Virasoro constraints sandwiched between any two physical states.  We classify solutions to SVC and show Hilbert space of a generic non-inertial worldsheet observer should contain extra states besides those of the usual Minkoswki worldsheet observer that is discussed in the string theory textbooks. These extra states break worldsheet parity or time-reversal, while canonical string states respect these discrete symmetries. We apply our formulation to an accelerated worldsheet with horizons, initiating the worldsheet formulation of a thermal string theory and strings probing horizon of black holes.