Hello I have little experience in this game but from my few matches this is what I saw works for me:
Fortify early against players even forts level 1 are nice choose 2-3 cities to focus mass production in them if your country get bigger more cities are added to the development list and 2 early baloons to scout early agression towards you.
What troops I feel worth spamming:
artilery mostly then if I see my enemy have fighters I build defending Fighters and if the enemy have impressive amount of art I go for bombers, about other units i feel the submarine is a nice unit if you know your enemy have a strong navy ( ik some might say bombers are better to deal with navy but I feel like submarines are more cost efficient than bombers. all the other troops seem quite not worth the time.
What do you think about my strategy? and also what is your favorite strategy?