Timbuktu was a paved city in the 14th century already (along with many other advanced developments such as plumbing and flushing toilets) - it opened it's first major university in 1330. Under the leadership of the Mali king Masa Musa it became the wealthiest city in the world. Musa himself still holds the record for the wealthiest person in history. Adjusted for inflation - he could have bought out Bill Gates's entire fortune about 200 times over.

There is less concrete evidence of the British destruction - which is not surprising, after all they were doing specifically IN ORDER to keep secret what had been there. Advertising it would hardly have been conducive to such an exercise, but records do exist of the orders given. There are photographs of the city taken before the British began their settlement which shows it intact.

And please note: Timbuktu was not a rare case. Just an illustrative example of typical British behavior. Hell they did the SAME THING in the boer war !