Theoretically, you could try to counter the Tower with Elementals. It's the only Castle that can offer some resistance. But, in reality, there's no good tactic for playing against the Tower, as the Castle can be developed quickly and turned into a fortress that enemies will break their teeth on. :)

In a short time??? Oh, please, don't even mention that! It's the longest and most expensive Castle to develop! Time is, unfortunately, what puts many people off from playing as the Tower....
In general, regarding the author's question – I advise you to read the discussion in several topics in this section... there are many tactics there. And in short – on such a large map, the two best options are Necropolis and Rampart with a hero specializing in Resistance. Both options are viable – but in the first case, you need to be able to recruit the maximum number of skeletons! In the second, you can win by developing faster at the start. And with the natural magic resistance of the warriors, enhanced by the hero's specialization. Well, I've already described the tactics – I don't see the point in repeating it!
As for the Conflux – IMHO, on the largest map, it has no chance against the Tower! Honestly... On a medium map, no problem... but here... no way :)
On small maps, Castles with fast development rule against the Tower – Barbarian, Swamp.