The atmospheric passage grave of Bryn Celli Ddu is well-known. There is evidence of interventions as early as 1699 when men broke in and were terrified when they found the central pillar looking ghost-like in the chamber.
The principal excavations were between 1927 and 1931 by W. J. Hemp. 
It is now thought to be a two-phase monument, originally a henge monument of the late Neolithic with bank (now gone), ditch and stone circle. Either as part of the ceremonies conducted at the henge or during its ‘re-dedication’, a decorated stone was raised. Also, a pit was placed centrally, a fire lit in it, and a human ear bone placed at the bottom before it was covered with a stone. Weird stuff! 
After this, the henge was ‘sealed’ by the building of the passage grave. This monument is one of the latest of this tradition known from Brittany to the Orkney Islands.
One of the chamber’s stones is decorated with an incised spiral. Both flint artefacts and human remains (burnt and unburnt) were uncovered during the excavation. Unsurprisingly, given how long exposure of the chamber, no in situ deposits were uncovered during the 1920s excavations.
Around the entrance of the chamber were hearths and quartz stones, together with the burial of an ox within a stone and timber enclosure, perhaps a ritual focus during the use of the tomb and/or after the blocking of the chamber. 
The Welsh Rock Art Organisation has explored the rock-art at the tomb and in the neighbourhood in some detail, as shown here. 
The passage and chamber have been reconstructed and consolidated with concrete and RSJs.
A section through the mound to the back of the chamber simultaneously gives the visitor a sense of the mound’s composition and allows light to enter into the otherwise dangerously dark chamber.
As I witnessed last year, the chamber is filled with all manner of graffiti, focusing on the easily inscribed concrete supporting beams and uprights. Some of the votive offerings – flowers, coins, headdresses etc, reflect the active use of this site for Anglesey’s neo-Pagan communities.
Archaeo𝔡𝔢𝔞𝔱𝔥 
I was here a week ago and was disappointed to see so much graffiti – the chamber and passage way had a lot of offering and I was told a shaman had been there ‘on a journey’ !
A few days before your visit a Druid musician had visited the site to draw inspiration for a song, she told me that quiet ‘connection’ within the monument would inspire her to write her song. Connection to the ancestors was an important element of her writing . Some Neo pagans then visit and leave without leaving a trace !
I think that is true for most visitors. Perhaps some find stimulus and inspiration of finding traces of previous visitors. Perhaps others find it insulting and annoying?
I think we tend to be offended by certain items – plastic , graffiti and paper rubbish and less by flowers maybe ? I would be interested in how many non Neo-pagans leave ‘offering’ in the way they do at wells and trees ? Yes some people find it very insulting and an eyesore and often it is
I think it is a category of activity where ‘belief’ and ‘ritual’ bleed into other forms of action. That makes it fascinating, and ambiguous