Short answer — as of current publicly available sources, Colombo’s one true integrated hotel + in‑house casino resort is City of Dreams Sri Lanka (the former Cinnamon Life complex) — the casino is operated by Melco and is inside the same integrated resort that includes the Cinnamon Life hotel and the Nuwa hotel. (Melco-resorts.com)

Notes and context

• City of Dreams Sri Lanka (Cinnamon Life / Nuwa at City of Dreams) — an integrated resort in Colombo with on‑site casino operations run by Melco (20‑year licence); the property includes the Cinnamon Life hotel and the Nuwa branded hotel with direct access to the casino. (Melco-resorts.com)
• Most other well‑known casinos in Colombo (for example Bally’s Casino) operate as standalone casinos or gaming venues in the city rather than being part of a hotel complex with a dedicated in‑house hotel casino. Bally’s is a major Colombo casino but lists nearby hotels rather than being an in‑house hotel casino. (ballyscolombo.com)

If you want, I can:

• Confirm the latest opening/operational status and hours for City of Dreams and other casinos (I can check real‑time/official pages).
• Find hotels that offer casino access packages or shuttle service to the major casinos in Colombo.