Egad!
Um...
15000?
Plate mail?
Well, luckily, plate mail isn't very effective against bullets (or some arrows for that matter) so... well damn, there are still 15,000 of them.
I would require 15,000 bullets if I were intent upon shooting every last one of them. That assumes I will not miss, and that double kills will not occur...
*thinks about solutions not problems*
Land mines, pit traps, and barbed wire for the first wave.
The XM307 for the second wave (a 1.00 caliber/25mm, full automatic, AMR)
A tactical retreat in an armored car with a dual mounted M61 Vulcan for the third wave.
Fortified battlements for a classic 'castle vs. attackers' setup. The difference being that my castle has mounted rocket launchers on the walls and the trebuchets launch grenade bundles.
Finally, the final worn survivors would be hunted down on horseback... with an Auto Assault-12 (AA-12) Combat Shotgun and a large satchel full of 20 round replacement drums.