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Unit - Create 1 Merc Compound for player 1 at (Start location of player 1) using default facing (No Options)
Now do this for each player and put it in the map initialization trigger and you are done! You might want to use offset by to make sure it doesn't spawn between the mineral line and the command center:
Unit - Create 1 Barracks for player 1 at ((Start location of player 1) offset by (1.5, 1.5)) using default facing (No Options)
Now do this for each player and put it in the map initialization trigger and you are done! You might want to use offset by to make sure it doesn't spawn between the mineral line and the command center: