Tower? would that be an observation tower? a gaming tower? how many ppeople allowed at once on it?
For sure you are gonna have to use 4x4, for the rest, 2x3 should be enought if done properly. Again, plywood is your friend. depending on the side, just plan on 2 sheets tick and it'll witstand 600lbs easy. Plan on reinforced X frames below and 45 degree angle 2x3 reinforcements if larger.
But to be honest... Towers are never a really good investment... everyone goes in 'cause its cool and then, everyone realise its a death trap and never go in after that... stick with bunkers
