I have explored my first 25 lands and upgraded my castle but I don't know whether to keep building forges or switch them to cursed foundries. I need advice please
Factories are a good choice, don't forget to have at least 1 Beastiary/War Aviary for an extra attack.
You might want to check out one of the stickies guides out there. They can explain it in way more detail than anyone just posting
I usually recommend exploring the first 4 highlands and building guilds on them as they are cheaper and give better plunder than subs, then go back to LL and convert to T3 ( subs, circles, aviaries, whatever you want). I do think it's a good idea to stick with higher attack (subs) so you can hit tougher and tougher EB's ASAP. This is assuming you've decided to go T3 and NOT hansel....
People, read Wulfs thread on Building and Land, Defensive buildings don't give extra 5% plunder. They give 5% more than balanaced or attack buildings......