Event - A unit is attacked
Condition - Attacked unit equal to Keeper of the Grove(example)
Action - Create 1 Footman at Position of (Attacked unit) facing default building degrees.
This'll created a leak in your map though. If you plan on playing it for about an hour it can cause lag, but if it's just this trigger on your map I wouldn't worry about it.
If you want to get rid of the leak, make a point variable and assign it to the position of the attacked unit and then use this custom script to remove it:
call RemoveLocation(udg_*insert Exact NAME HERE*)
Spaces = _ in the Custom script.
Hope I've helped,
Beetlebomb