from the description, it's not clear if the modifiers are applied to the hitpoints, the power levels or to the actual resulting power of each team (the sum).

A third function parameter, terrain, determines which side has an advantage in the fight: "island" increases humans' power by 20 points, and "sea" increases the primates' power by 10 points

same about Jane

• the description of the objects do not match the actual property names: hit_points instead of hitPoints, same about Jane stuff
• not a problem, but the sample tests (maybe fixed too) use unnecessary parentheses for the expected result. It's not really "pretty" to look at...

Q: why the power levels aren't defined as a global variable? If not, at least put them in lowercase in the description, so that they match the properties.

edit: prefer multiply with to multiply against