Tamper-respondent assemblies and methods of fabrication are provided which include a tamper-respondent electronic circuit structure. The tamper-respondent electronic circuit structure includes a tamper-respondent sensor. The tamper-respondent sensor includes, for instance, at least one flexible layer having opposite first and second sides, and circuit lines forming at least one resistive network. The circuit lines are disposed on at least one of the first or second side of the at least one flexible layer, and have a line width Wl≤200 μm, as well as a line-to-line spacing width Ws≤200 μm. In certain enhanced embodiments, the tamper-respondent sensor includes multiple flexible layers, with a first flexible layer having first circuit lines, and a second flexible layer having second circuit lines, where the first and second circuit lines may have different line widths, different line-to-line spacings, and/or be formed of different materials.