The Millenium Centre was a compromise reached after the spectacular failure of an international architectural competition for a new opera house http://en.wikipedia.org/wiki/Cardiff_Bay_Opera_House ), neither the UK funders nor the Welsh being ready for the architecture of Zaha Hadid http://www.zaha-hadid.com/architecture/bmw-central-building/ ). Much as I have been sickened by the abuse of architectural competitions, I have to agree that in this case, the final muddled outcome is better than the competition would have yielded. For one thing, it seemed to be the only place on the waterfront with easily accessible loos and once in, then firmly hooked.
The facade is a massive tribute to Wales as traditional land of song and poetry. The words , by Welsh poet Gwyneth Lewis. The Welsh version is Creu Gwir fel gwydr o ffwrnais awen, which means "Creating truth like glass from the furnace of inspiration". The English is In These Stones Horizons Sing. I think the literal translation conveys more meaning, but it would have added an extra storey.