I feel like this game is missing something. I know! Practical resources to upgrade the defense.
Personal list of upgrades:
Spike walls can prevent arrows from hitting units, can cause small thorn dmg for every attack, and repaired after battle (not during battle).
Increase max capacity of each post.
Use slaves as literal meat shield(whether they're or not pregnant, also helpful for removing slaves with bad traits). A bit dark, but makes sense in fantasy war
A clinic for healing wounded units (Requires a cleric slave)
That is all I could brainstorm for now.