How did the Archimedes crater fill with lava? This is the Archimedes crater, on the moon, near the Apollo 15 landing site.

It is located in Mare Imbrium, a plain filled with lava.
The strange thing is that the interior of the crater looks flat, as if it had been also filled with lava, but there is no passage for lava to come inside the crater. All the walls appear to be intact.
So how could lava have possibly entered the crater?
Could it have had such low viscosity that it infiltrated as seepage?
The interior of the walls appears to be broken as if they suffered a landslide.
(The image source is this video, apparently a Lunar Reconnaissance Orbiter image)
Great question! The mare lava bubbled up through fractures in the floor of the crater, burying its ring system and central peak.