Cerro de la Cruz means Hill of the Cross, and there is indeed a cross at the top of this hill. But the main reason for climbing it is to get a spectacular view of Antigua and the surrounding mountains, including a couple of volcanos. It's an especially good vantage point for watching Volcan de Agua vent its steam every 20 minutes or so. It's a good idea to make this one of your last stops in Antigua, so you can view the layout of all the places you've already seen at close range.
If you go:
The park is closed from dusk to dawn out of safety concerns, but is safe at other times. The ideal time to go is at dawn.
Links:

H78C+25Q
Antigua, Sacatepéquez 3001 Guatemala
info@inguat.gob.gt
Phone Numbers:
Guatemalan Tourism Institute
+502 (2290) 2800
+502 (2290) 2800