That stone tower is your new swamp base. There's almost definitely a spawner
inside. Destroy that of course and build a portal, bring in stone/cores and smelt
all the iron ore you're gonna find right there. Make a stonecutter first and
repair/finish/expand that tower. If you find enough iron you can upgrade your
forge (don't forget 6 copper) and fully upgrade whatever iron stuff you make.
A swamp base is also nice to farm leeches, entrails, chains and root monsters.