To find the secret portal in the Bathhouse Battle level of Astro Bot, follow these detailed steps:

1. Reach the Area with the Cherry Blossom Tree:
– After climbing up the pile of sponges, soak up more water to become big and powerful.
– Head left towards the big red torii gate.

2. Climb the Cherry Blossom Tree:
– Jump onto the cherry blossom tree and climb up.
– Make sure to spray water on the flames coming out of the nearby chimney to extinguish them.

3. Enter the Chimney:
– Once the flames are out, you can safely jump inside the chimney.
– Be cautious of any enemies inside, such as a screw enemy, and defeat it to proceed.

### Summary

To find the secret portal in Bathhouse Battle, you need to:
1. Climb the Cherry Blossom Tree: After soaking up water, climb up onto the cherry blossom tree.
2. Extinguish the Chimney Flames: Spray water on the flames coming out of the nearby chimney.
3. Jump Inside: Once the flames are out, safely jump inside the chimney.
4. Defeat the Enemy: Be cautious of any enemies inside and defeat them to proceed.

By following these steps, you will be able to find and access the secret portal in Bathhouse Battle.