I second the suggestion to give you visibility to Bourne from the beginning. I found it after a bit of scouting with Floating Eyes, but especially novice players may not come up with that, and it was pretty disorienting to try to arrange your defences before you even knew what direction you should be defending.
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On the first try: I managed fine in the beginning, but soon the enemies started slipping past my lines and I had to break ranks in order to stop them. This was made worse by the fact that somehow I forgot that the Wraiths are summons, and so I sent my mages to a desperate rush after them... forgetting that most of them would have vanished before reaching Bourne anyway. I managed to rack up the 100 kills, but after that the rest of my main force collapsed, wiped out by those Beasts of Agares and whatnot.
At the same time, a crack team consisting of the archmage, Guybrush and a single Radiant Guard had reached Bastradam. Thankfully, by placing my units appropriately, the Balors spawning in the city just ignored them and rushed towards Bourne while I kept bombarding the actual defenders with fireballs and elementals. I managed to destroy the city, then took the reinforcements I got and went looking for other Infernal cities. By the time I reached the second city, most of the team had been wiped out, leaving only Guybrush, the Archmage and a single Confessor left. Then I spent quite many turns bombarding the city with a Fire Elemental, slowly wittling away the defenders and the units that kept spawning there.
I finally managed to take it down, but lost Guybrush in the process. Before I could move on to the next city, I got a message saying that Hyborem had won a conquest victory, so I suppose he had broken through Basium's defences. A pity, it would have been an epic story had I succeeded.