“...I’ve done ten days of my garrison duty...”
Under the feudal system land was granted in return for a fixed number of days of military service. The Lord had a right to demand forty days from his men each year. Our archer owes service to the Lord of Carew, who owed it to his lord of Pembroke.
“...You don’t need many men to hold a castle...”
A well-designed castle could be protected by just a couple of dozen men against a much larger enemy. Conwy Castle, for example, listed a permanent peacetime garrison of only 30 men when it was built, although this was increased if enemies were in the area. Pembroke, with its natural defences, was never taken until the time of Oliver Cromwell and gunpowder.
“...I practice regularly...”
Longbowmen were highly trained, and needed constant practice to keep their muscles and technique in good condition. Later medieval armies often preferred crossbowmen, who needed little training by comparison.