(Disclosure: This site contains product affiliate links. We may receive a commission if you make a purchase after clicking on one of these links. Read our affiliate disclosure for more info.)
The tower defense / turret defense game Sol Survivor is on sale at Steam for $4.99 (50% off). A four pack is also on sale for $14.98 ($3.75 per copy). This deal runs until May 6th at 10AM Pacific Time.
This game is supposed to have a really good coop mode.
There’s a demo available on Steam.