Probably the most famous door in the country?

And I walked in through it today!
Had a fascinating tour organised for City of Westminster Guides. Sat round the cabinet table, sat in a £130k William Kent chair, met Larry the Chief Mouser. Walked up the stairs past pictures of all previous prime ministers. Realising that there have been 12 in my lifetime (not counting DC - he doesn't feature on the stairs as he's still in office) made me feel quite old!
No photos allowed inside though, as might be expected. All phones and cameras had to be left in pigeon holes just inside the front door.