I visited in April 2024 the DOMunder experience in Utrecht. Like previous reviewer has explained, you get to go under the Domplein square to explore with flashlights the ruins of the Dom Church's nave that was destroyed by a tornado(!) in 1647. However, besides the church ruins you can see also other ruins from different time periods, one of them being the Roman period. There is a very tiny part of the Roman wall on show as the Roman ruins are not the main focus of the DOMunder. 

The DOMunder tour started with a presentation in the basement of Utrecht Music School where the guide goes through also the Roman history of the Domplein. And here you can get really close to the actual Roman wall ruins! Before the presentation started our guide showed us a hole in the wall where you can see and touch the original Roman wall. This is not automatically part of the tour so you can ask from your guide to see it in case they forget to show it. I really enjoyed the DOMunder experience and can recommend it for anyone interested in history.

I booked the tour online and checked in for it at the Utrecht tourist information center. From the tourist info I got myself a free brochure about the Lower German Limes sites around Utrecht. They all seem to be worth of visit if you're interested in Roman history but the DOMunder is the only way to see the original ruins in Utrecht. Around the Domplein, metal plaques mark where the walls of the Roman fortress once ran (or still run underground, pictured).