Why pre-Roman archaeology is hard to recover at Portsmouth

Has any work investigated pre-Roman defences at Portsmouth, where Roman and later defences occupy the same ground?

Portsmouth is a conflicted archaeological area: successive layers, an active port, and public and government use limit access to earlier remains.

Portsmouth contains overlapping archaeological layers that compete for the same ground, making interpretation difficult even before excavation access is considered. The modern port remains heavily used and has extensive public and government involvement. Archaeologists can investigate some areas, but they cannot open every promising site simply to look for a pre-Roman predecessor. Those limits leave the question open without claiming that a specific earlier fortification survives.