How come my Troops died while attacking, even though I have plenty of room in the Hospital?

Troops can only be hospitalized if they are defending your Castle. If Troops are deployed (either to another Structure or attacking), unless "Wings of Protection" is used, they will always die. By tapping on the Hospital building from your castle screen, you will find your Injured Troop Capacity. However, this number is further broken down and divided between different Tiers of troops. If the capacity in a single Tier is full, even if the Hospital itself has capacity left, additional Injured Troops of the same Tier will die.