My siege towers always burn, every single time. The same with rams.

Most would disagree but I would stick to ladders. You lose a ton of people to arrow fire but those are easily healed.

One key thing though: Once the ladder is attached to the wall, you have to wait till the first few men are near the top. Then select the unit and attack the defenders. This might have had a huge impact on why your spearmen were killed.

The difference is: by just right clicking the wall, the soldiers will climb the wall and assemble in formation at the top. They will try to do this even though they are attacked. While some will fight back, the majority will just stand there or walk through the enemy and die.

By right clicking the defenders, you're telling them to kill any on the wall, not form-up at the top.