Abatis take out calvary. If you attack without calvary, abatis will not harm you.

An abatis is a means of defence formed by a line of fallen trees, with sharpened branches and most often enhanced with barbed wire.

Abatis

abatis

You need the siege engines to reduce fatalities in your army. The battering ram and the catapult are effective weapons against the abatis. Be sure to build many workers and warriors because they are the "cannon fodder" against the wall defences. They will be used first, so this is one strategy to use against the walls.