I can see where you're coming from in regards to the monument possibly being a 'crypt', but I think it has more in common with the Ara Pacis Augustae than anything else. The stairs on my example are indeed worn away but can be seen on better preserved specimens.
I'm not too knowledgeable about this issue but I'm glad to learn. Now I also learned something about the Ars Pacis.
With some apologies for my poor photography I thought I would share my one and only Cistophorus. Marc Anthony and Octavia . Conjoined heads of Marc Anthony and Octavia. Reverse Dionysus holding cantharus and thyrsus, stg.1. on cista mystica flanked by two snakes. RPC 202, CRI 263,CRR 1198. NRSC Sear 1513. Ephesus Summer-Autumn 39 BC. 10.88mm 11.68 gms.