To defend an ordinary Castle Age Knight raid, move threatened villagers to safety, use early walls and Town Center garrisoning to buy time, then guard exposed resources with Spearman-line units. Scout again before committing: a Feudal Stable does not confirm Knights, and ranged support changes the response.
Scout the age and the army
A Stable is a clue, not confirmation of Knights. The official lessons place Stables in Feudal Age and Knights in Castle Age, so a Feudal Stable can produce a different threat first. Scout again as the opponent advances and watch the units that move toward your base. If you see another military building, prepare for the possibility of a mixed army. Building choices suggest a plan, while the units you can see tell you what needs answering now.
Protect villagers before and during the raid
Use your buildings, nearby forests and walls to make entry routes harder before the attack. The official defense lesson says walls take time to build and work best when started early. If Knights get through, move villagers away from the threatened resource and garrison those who can safely reach a Town Center. Garrisoning protects them temporarily but interrupts their work. Move them back to gathering when the route is safe, and remember that a Town Center cannot cover every exposed resource area.
Place the Spearman line where it matters
The official military lesson describes the Spearman line as a counter to most cavalry. Train and position these units near the openings and resources the Knights can reach. A few isolated spears cannot guard every part of an open base, so judge how much ground you need to protect. Spearmen themselves use food and wood, but official balance notes show that the Pikeman upgrade can also cost gold. Check the current technology tree for your civilization's available upgrades and their costs before committing resources.
Reassess when the enemy adds support
If the enemy adds Archer-line units, a spear-only answer becomes less reliable: the official military lesson says the Archer line counters most infantry. Protect your villagers while you decide how to answer the ranged support. Monks can attempt conversions, and some civilizations have Camel Riders, but their usefulness depends on the army you face, the units you can access, your resources and your control. Identify mounted ranged units separately. Mounted Crossbowmen fire at range from horseback; they are not ordinary Knights.