Betimi is correct. You do roll once at each level for hit points and the minimum is half plus one. So characters who roll a d4 will have a minimum of 3; d6 = minimum of 4; d8 = minimum of 5; d10 = minimum of 6; d12 = minimum of 7.
Now my opinion on Armor/ Gauntlet Spikes of Defending. This has come up in my real life game (by me actually) so I have researched this and there are some concerns. In the description of Defending it states, "the wielder." Wielding means to hold and to use. Some believe that simply wearing spikes and calling upon one of the enhancements is not Wielding the spikes. The enhancement enhances the spikes so to activate the enhancement you actually have to wield the spikes. So technically you would have to either be attacking with only your spikes or two-weapon fighting using your spikes and another weapon. The other issue is balance. Adding an armor enhancement is very cheap. Compare the armor enhancement to other items such as a ring of protection or Bracers of Armor. The Armor Enhancement is half of the cost of protection from the RoP and equal to the bracers but you can only ever wear one BoA or RoP. You could add spikes to your armor, gauntlet, and shield. So you could very easily stack your armor to the point where you can only be hit by a natural 20. Fun for the player but not so great for game balance or for the DM who will then have to place creatures against the party that actually pose a challenge to the one or two players with the 30+ armor class while the other players get squashed. So I (or we) decided to compromise by not allowing the Defending enhancement to stack. So you may place the enhancement on your spikes but multiple defending enhancements will not work together. I may create a Greater Defending enhancement that will cost +2 or +3 that will allow the Defending to stack but even in my real life game I haven't needed to yet because even our 12th level characters rarely are hit by monsters due to their A.C's that range between 30 to 35 and honestly with buffs I could easily boost my Cleric's A.C. even higher but as of yet, I have not had any need to.
I hope this seems like a fair compromise.
Now my opinion on Armor/ Gauntlet Spikes of Defending. This has come up in my real life game (by me actually) so I have researched this and there are some concerns. In the description of Defending it states, "the wielder." Wielding means to hold and to use. Some believe that simply wearing spikes and calling upon one of the enhancements is not Wielding the spikes. The enhancement enhances the spikes so to activate the enhancement you actually have to wield the spikes. So technically you would have to either be attacking with only your spikes or two-weapon fighting using your spikes and another weapon. The other issue is balance. Adding an armor enhancement is very cheap. Compare the armor enhancement to other items such as a ring of protection or Bracers of Armor. The Armor Enhancement is half of the cost of protection from the RoP and equal to the bracers but you can only ever wear one BoA or RoP. You could add spikes to your armor, gauntlet, and shield. So you could very easily stack your armor to the point where you can only be hit by a natural 20. Fun for the player but not so great for game balance or for the DM who will then have to place creatures against the party that actually pose a challenge to the one or two players with the 30+ armor class while the other players get squashed. So I (or we) decided to compromise by not allowing the Defending enhancement to stack. So you may place the enhancement on your spikes but multiple defending enhancements will not work together. I may create a Greater Defending enhancement that will cost +2 or +3 that will allow the Defending to stack but even in my real life game I haven't needed to yet because even our 12th level characters rarely are hit by monsters due to their A.C's that range between 30 to 35 and honestly with buffs I could easily boost my Cleric's A.C. even higher but as of yet, I have not had any need to.
I hope this seems like a fair compromise.