Tactics against Necromancers.

As they say, it all depends on luck and the size of the map. If the map is small, then it is indeed better to take a Stronghold (but it's better to upgrade the birds). If the map is large, then you can choose a Castle, Tower, Rampart, or Dungeon, but there's a catch: you need magic, and more of it (especially witchcraft). Ideally, it's Conflux – firstly, you'll always go first (even if the Necromancer has Azure Dragons), secondly, it doesn't retaliate, and thirdly, the city itself is strong, and when there are many fairies, they effectively destroy skeletons and vampires!