I notice many people don't know this. If you place a necromancer and don't have enough coins to fully upgrade it, you will run a risk of it being permanently stuck on a lower level. This happens when the iron golem comes out at level 1 or 2. Since it doesn't despawn, it will be forever a level 1 or 2 golem. In order to prevent this, it's best to have 2300 coins to cover both upgrades prior to building, and also build when there are no mobs around. It's a minor setback, but having a level 3 golem will help greatly.
