A detailed study of the structure of the doubly mid-shell nucleus ¹⁷⁰₆₆Dy₁₀₄ has been carried out, following isomeric and β decay. We have measured the yrast band up to the spin-parity Jπ=6+ state, the K=2 γ -vibration band up to the 5+ state, a low-lying negative-parity band based on a 2− state that could be a candidate for the lowest energy octupole vibration state within this nucleus, and a candidate for the Kπ=6⁺ two quasi-particle isomer. This state was determined to have an excitation energy of 1643.91(23) keV and a half life of 0.99(4) μs, with a reduced hindrance for its decay to the ground-state band an order of magnitude lower than predicted by NpNn systematics. This is interpreted as being due to γ -vibrational mixing from a near degeneracy of the isomer and the 6+ state of the γ band. Furthermore, the parent nucleus ¹⁷⁰Tb has been determined to have a half-life of 0.91(ˍ₁₃⁺¹⁸)s with a possible spin-parity of 2¯. ; This work was partially supported by the JSPS KAKENHI (Grant No. 25247045) and the National Research Foundation of Korea (Grants No. NRF-2009-0093817, NRF-2013R1A1A2063017, NRF-2014S1A2A2028636 and NRF-2015R1D1A1A01056918). OJR acknowledges support from Science Foundation Ireland under Grant 12/IP/1288. Work at ANL is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Contract No. DE-AC02-06CH11357. GL and PHR acknowledges support from the UK National Measurements Office.