Just as the Luna Temple built off Santorini by Alexander the Great is fictional ... so, I'm afraid, are all claims Alexander was buried at Amphipolis.
Based on what I know, I think it could well have been started by him and finished by the Antigonids; they could well have left it empty, assuming that they would 'soon' bring his body back from Alexandria ... but they never did. Then it would have been either re-used for a subsequent ruler's burial or possibly kept as a cenotaph / heroon to his cult, possibly jointly with Hephaestion, as they were sometimes honoured jointly as dioscouroi.
Are there more chambers? These diagrams are very useful for showing that the three chambers so far identified don't go very far into the mound, suggesting that there were. It is very unlikely that there was a chamber at the centre of the mound, since that was the support weight for the lion, but since the architect made some structural mistakes, anything is possible.
If there are only three chambers, since these are close to the edge of the mound, the weight they carry is lighter and so they should not have the structural issues we are seeing. I suspect that there are more chambers, in worse condition, and that there is a sort of domino effect, with the badly damaged inner chambers pushing outwards onto the third and second ones ...
People have asked about earthquakes. This damage could have been caused by an earthquake, but the removal of the superstructure was deliberate - we know that as the parts of the lion and the base were found some distance away, by the river, and the reason they were not originally associated with the mound was because of this.
The very steep steps down are highly unusual, and I am surprised they have not attracted more comment as there are few parallels.
The gap between the spinx gate and the steps was rather narrow, and then we have to remember that there was a later wall added.
The floor actually looks like this, and has a pattern that echoes the masonry lining the walls:
Again, this is the official reconstruction by the archaeologists:
Castel Sant'Angelo; the original complex was slightly bigger, and closer in size to the Mausoleum of Augustus ... but still tiny compared to Amphipolis!
Although there are almost a dozen earlier more-or-less round buildings, the perfect circle is associated with Dinocrates by the archaeologists working there.
I can't discuss any geo-phys surveys, but I am not expecting gold and treasure. We hopefully will find some things left behind, but the fact that finds have not been announced from the back-fill suggests that they were probably removed when the tomb became structurally unsafe.
Below is a plan of the mound over Tomb II at Vergina. There are a variety of interpretations of who was buried there, but in my opinion Philip II is the best candidate. You can see the plan of a small shrine or naiskos to the right, which may have been a cult shrine.
Vergina was sacked by the troops of Pyrrhus in one of the many wars fought by the successors of Alexander, and the mound seems to post-date this sack since it encompasses several tombs, unlike Amphipolis where the mound is part of the original structure. Various finds in the mound at Vergina come from the funerary pyre, suggesting that there had been a smaller mound before.
The destruction of the Lion and superstructure was not in an earthquake, nor does it seem to be Christian iconoclasm as previously thought. The archaeologists shifted the date downwards into the Roman period, based on small finds such as dated coins found in this destruction layer.
Why someone with a great deal of power would put so much effort into destroying and concealing the tomb is very puzzling. It seems to have been an officially sanctioned project, since although mobs could destroy buildings it is very unlikely that a mob moved the blocks such a great distance.
The destruction could be key to identifying who was buried at Amphipolis. If it was someone like Hephaestion, then it may have been because an emperor did not approve of him? Or it could be linked to a megalomaniac such as Caracalla - perhaps he wanted the only tomb linked to his beloved Alexander to be in Alexandria? There are as many possible answers to that one as there are theories!
Yes, the way the margins are drafted on the masonry in the entrance is quite unusual, but not without parallels - and the whole point of exceptional and important buildings is that they often have unusual architectural features ... that's what makes them special. For example the carved column drums at Ephesus (and I hope no one is planning to re-date that to the Roman period).
There are rosettes carved at Amphipolis, but these are not, to the best of my knowledge, a specifically royal symbol, and they can be found on the funerary stele of ordinary ancient Greeks.
As I've said before, the archaeologists working at Amphipolis are very good. I've also pointed out that any semi-competent archaeologist could make the observations I'm making. I am competent, but unfortunately not all archaeologists are.
I have to go walk the dog and run errands, but I'll try to do another post later today answering the many other good questions people have asked. Meanwhile I highly recommend looking at the inter-active floor plan of the excavations at The Amphipolis Tomb web site here.