Common Towers Strategy in Soulkeep
I have said that you need to get more than 2000 common fiends soul and this is the largest number as rare and epic one is not much and less than that of common by far way so you have to consider this when you play as you have common fiends and also have rare one but less and some few epic.
So what I have to say is that you need to have more and more defense against common cards as in every wave and every battle you have to play against high number of common one and in this case you don't need epic towers or even epic spells.
All you have to do and focus in to have common towers and those should be the main choice but common towers are weaker than those of rare and epic but they are high in numbers as you have to face more than 400 every battle.
So I recommend to use common towers in high number also and this would be your first step in any battle to nearly fill every free position in the battle with a common tower then you upgrade this tower and this is according to what you need if you want high damage or more range or speed and this is according to mana available and also tower position.
So Common Strategy is the main one to be used in any battle at league one and you can use recommended towers and spells that are from the game or you could choose your own after trying and playing the level.