Tactics for the Swamp (Fortress).

The difference between building Wyverns in a Castle and Hydras in a Fortress is only 2,500 + resources. That is, you can only buy three Wyverns with that amount.

The difference becomes more noticeable when you encounter an opponent on the second week, and you only have 2 Hydras, which can be easily killed by a Valkyrie (she might not even need a new wave of units).