Tower Bridge is London's most iconic Victorian drawbridge — the one everyone pictures when they think of the city. You don't need to pay to experience it.
Tower Bridge is the actually impressive one — not London Bridge, which is just a normal-looking crossing further down the Thames. The bridge itself is free to walk across, and for most visitors that's the full experience. The paid Tower Bridge Exhibition takes you through the glass-floored upper walkway and into the engine rooms, which adds context but isn't essential.
Walking across it for free is absolutely worth it — it's one of the most emblematic views in London. The paid exhibition inside is enjoyable but not essential.
Walking across is free. The Tower Bridge Exhibition — which includes the glass-floored upper walkway and engine rooms — is ticketed; prices move, so check the official site before you go.
Walk across it for free, ideally early in the day before crowds build, or take a river boat along the Thames and sail under it — that's an angle photos from the bridge itself can't give you.