In Age of Darkness: Final Stand, you will have to rely on your champion and your soldiers if you want to survive the night. Your champion will level up by killing nightmares, but your troops will need to be upgraded. You have a skill tree that allows you to upgrade them.
This skill tree is located in the bottom center of your screen and is the top icon that looks like an arrow. Click this, and it will bring up the skill tree for your melee, ranged, and siege units. Each of these upgrades requires a set amount of resources, and this number increases as you progress through the tree.
Consider carefully which upgrade you wish to select because some options may become locked from you as you progress through the skill tree. It may be tempting to grab whichever upgrade you can afford at the time, but you should consider what your army lacks.
Most upgrades require wood, stone, gold, or iron. However, the final upgrades for each unit type require Dark Essence. This resource is not easily come by, so you should take extra care before spending it. You should focus on your melee and ranged units early on when upgrading, as siege units are not available until later.
For more information on Age of Darkness: Final Stand, check out Early game tips in Age of Darkness: Final Stand and What are the Horrorified and Embolden effects in Age of Darkness: Final Stand? on Pro Game Guides.