Play with the ranges...mortars have far greater range then redoubts, but still your mortars are on the outside and your redoubts are on the inside. If you switch that, you will be able to keep firing with mortars while they struggle to get your redoubts down.
2nd, try to make crossfires. Meaning try to keep defenses that shoot seperated enough so that an opponent cannot shut them up all at once using a tactic, but close by enough for multiple buildings to fire at the same bunch of attackers....for example again the mortars. They have great crossfire abilities which you do not take advantage off...
Tanks and heavy horses tend to go for towers first if there are other buildings next to it, so if you have cannons besides towers, they will attack towers and will be slain by your cannons...try to investigate which troops are going for which kind of buildings first...and use that