Aristo had a breathable atmosphere but was 90% covered by acidic seas. The land was arid but supported primitive plant life. At the time of the Liberator crew's visit the sea levels were slowly rising. The most notable native life form was the Phibians, aggressive semi-humanoid creatures with reptilian characteristics.
At some time the planet had held an advanced civilisation but the remains of their cities were now mostly underwater or subterranean. Ensor appears to have based himself in one ruined tunnel complex. Phibians infested at least some of the ruins.
Aristo was largely unknown to the Federation with only a single exploratory mission having been sent there in the past. (It therefore seems most likely that the civilisation of Aristo was wholly alien rather than being a lost human colony.) Servalan and Travis found the remains of the expedition members in the tunnels near Ensor's base; they had fallen foul of the ferocious Phibians. (A: Orac)