Infirmary

When troops are wounded in Expedition mode combat, they are sent to the infirmary for healing, provided there is capacity in the Infirmary. If additional troops are wounded beyond this capacity, they become permanent losses to your account (although some share of those losses can be recovered at the Enlistment Office building).

Therefore it is important to make sure that your Infirmary is of a sufficient size such that you are able to absorb all of your expected injured troops. The amount needed will vary depending on how much player versus player (or PvP) content you choose to partake in, or how lawless your Empire political dynamics tend to be.

In any case, upgrading the Infirmary will increase the Infirmary Capacity only, it will not increase the healing speed of troops or reduce the healing costs. There are other ways you can improve these things, but they require completing research from the Academy and the War Academy. Additionally, you can further improve your total Infirmary Capacity with the “Ward Expansion” series of research projects in the Academy.

The infirmary is not required to advance past level 1 as a prerequisite for Town Center levels, so you can safely deprioritize it versus other buildings if you feel like you have sufficient capacity.

Finally, you can enhance the speed at which you heal troops in the Infirmary through a technique known as batch healing. Check out this guide to find out more about it.