lanun assassin question

[Emptiness]

summons have upkeep too don't they?

Yes, but they can be instantly resummoned each turn. This means you have to keep fewer expendable units around, and pay less upkeep. For example, say you are going to move your stack through dangerous territory, and 3 Assassins attack each turn, and you are exposed for 10 turns. Using Skeletons to protect your casters would require 3 casters to resummon Skeletons each turn, and cost a total of 30 upkeep. Using Scouts would require you to start with 30 Scouts and would cost a total of 165 upkeep (30 + 27 + 24 + ...).