You searched for: “fortressing”
fortress, fortresses; fortressed, fortressing (verbs)
To protect by or to furnish with a strong defensive structure: "Many geographical areas in history have fortressed their villages, towns, etc. so they could protect themselves from invaders who wanted to steal their property and even force them into slavery."
"The people felt the need to fortress their town by constructing a fortress so they could defend themselves against attacks by wandering groups of thieves and killers."