Galle Fort is either a blessing in disguise, or a tourist trap, it all depends on what you want. If you're going to Sri Lanka to experience local culture than Galle may not be for you. If you need a break from the hectic pace of Sri Lanka, than it may be nice stop. Galle Fort is like a slice of Europe in Asia, with tourist shops and restaurants that look more Greek or Italian than Sri Lankan.
Galle Fort was originally built as a Dutch fort. It still has a great deal of the charm left from its past, most notably a giant wall that surrounds it. Many of the shops and hotels are in the original Dutch buildings, and can sometimes be very grand. You will pay for this grandness if you eat or stay in this fort as prices are much higher than you'll find no more than a 20-minute walk outside the walls.
On weekends and evenings you'll also find many Sri Lankans enjoying the local atmosphere, particularly around sunset. At sunset everybody collects at the wall near the sea to watch the sunset, and maybe even jump into the water to cool off. Then some people go home, and others enjoy the European feel of this Dutch fort in Asia.