Tutorial-type elements and (in-level-)storyline are repeated every time you restart a level. Gets annoying when optimizing tactics.
As mentioned elsewhere, infantry outperforms (much more expensive) artillery.
Also, especially when using artillery, enemies' health bar sometimes behaves erratically -- going up again for no apparent reason after rounding a corner, then drops from ~ 100% to zero even though dozens of hits haven't made a dent thus far.
Tactics of artillery is counter-productive -- always engaging "youngest", fittest group of attackers first.
You have a long way to go. Keep going, looks like it will be nice when done.
HTH