Both the Clay Golem/Mindflayer group and the Fire Hydras become common on this floor, but the Soldiers and Rakshasas are gone, so that's just fine. Dark Flans make a return as an uncommon encounter, and Dark Warriors are a bit more likely the show up. The real threat here is the rare encounter with Warmech, who is basically this game's optional super boss, except that you may run into it whether you want to or not. It has the same 2,000 HP and is just as well-defended as Tiamat, except it can dish out a lot more hurt much faster. Just pray that you don't see it unless you're looking for it. Check out the Warmech page for specific strategies.
The top floor of the Floating Castle is very simple: it's just one long bridge directly to TIAMAT. Most of the battles here are straightforward, but you have a 4.7% chance per encounter to run into WarMECH, the "super boss" of FF1. You can run if you're not prepared to fight.