This dataset accompanies a study of the room acoustics of the St. Nicholas Chapel(Nikolauskapelle), a late-Gothic, acoustically coupled two-storey hall chapel ofAachen Cathedral (volume ≈ 3050 m³, mid-frequency reverberation time ≈ 4.6 s). Itprovides the primary measurement data, derived results, and analysis codeunderlying the associated journal article (Zerwas & Kayku, Acta Acustica). Contents:• Measured room impulse responses — 46 four-channel B-format (FuMa) impulse responses at 48 kHz, recorded with a Sennheiser Ambeo / IRIS 3D system using exponential sine-sweep excitation, for three source positions (main room, side aisle, gallery) and up to 22 receiver positions per source, following ISO 3382-1.• Derived room-acoustic parameters — 19 ISO 3382-1 parameters (EDT, T20, T30, C80, C50, D50, Ts, G, lateral fractions, etc.) per receiver in 29 one-third-octave bands, plus speech-intelligibility quantities.• Background-noise measurement — one-third-octave quiet-state spectrum (NTi XL3) at the central main-room position.• Pink-noise reverberation-time cross-check (NTi XL3) at selected positions.• Multi-exponential (multi-rate) decay-analysis results (DecayFitNet: T1, T2, A1, A2).• Simulation outputs — impulse responses and parameter exports from a geometrical-acoustics model (ODEON) and a hybrid wave-based/geometrical model (TREBLE), together with the material/absorption data used for calibration.• Analysis code (Python) used to process the measurements and reproduce the published results. Not included: the 3D geometry is derived from a third-party laser scan of AachenCathedral and is not redistributed here (available from the laser-scan authorsunder their data-sharing agreement); the European church reverberation-timecomparison data are third-party (Peutz Consult GmbH, personal communication) andare not part of this deposit. The measurement and result data are released under CC-BY 4.0; the accompanying codeunder the MIT License. Please cite the associated article and this dataset.