Build your castle using five stone and three wood. When it has been built, extend it for a couple of minutes by way of a few extra walls, a gate house, and a bit of a moat. Click on End Design, and demolish it straight away for 65,000 stone and wood.
Wait for a merchant to arrive. Sell 100 stone four times, then buy 2,000 of each weapon. Your gold will increase rather than decrease - and as you can see this is quicker than constantly selling stone or wood. But don't buy more than 2,500 of each, or you'll start losing money big time.