This means you can have at most five different unit types in one army. Mixing undead can be particularly bad, unless the hero has a good leadership secondary skill. Luck can also bless, or hamper, an army, and it pays to visit shrines to boost your favour with the gods. Each has three levels of knowledge - basic, advanced and expert which will offer increasing benefits. There's a similar screen for town and castle summaries.
Heroes can learn spells.
A castle's mage tower will have a number of spells available depending on how many levels it's had added, and not all spells of all levels will be accessible. A level 2 tower may only offer three level 1 and level 2 spells, while there are around 12 spells in the game at each of those levels. The most powerful level 5 spells include dimension door, resurrection true, summon elementals and armageddon. Once expended, spell points return with time, or via visits to mage towers or magic wells. Finally, a hero can collect artifacts found on the game map.