This was MTG-inspired, correct? (There are cards there too that automatically come into play if in your starting hand)
Nope, though that does bring up a talking point that has bugged me. Other TCGs take up so much design space that there is some inevitable overlap, intentional or not. If I were purposefully making a card like a card from another TCG I'd consider myself a poor designer because I'd not really be designing as much as copying. I'm perfectly ok with it legit being like another card in a well designed game though since that means I accidentally came across potentially good design space.
The real thought process was a high risk high reward anti-aggro card. The reason I went with basing it off of Titanium/Diamond Shield is because of both the simplicity and raw power it grants as a control card. The pricing took a lot more thought since I didn't want to make it too close to the others but what I finally settled on was that with a 2 pillar start (what I'd consider average with most deck builds), and a matching mark, assuming you play nothing else the shield comes down on turn 3. My rationality is that this is either 3 times as fast or twice as slow which seems to be a fair trade-off. However....
Tower Shield only provides 1 | 2 Defense and costs 4 | 5
Also seems reasonable since you're trading some of the protection for less risk.
the diamond shield we have now is a kite, more suitable for blocking meele attacks (if im not mistaken) A tower shield was more effective when fighting arrows, and aerial assaults, thus why not have it block 1/2 and 3/4 on air borne creatures. It would make more sense, and be more effective VS earths arch enemy air.
This with the free cost (the whole reason the card exists) would be a wordy nightmare.
For the sake of balancing the card pretend it doesn't have a name. Name and flavor can be changed, mechanics are the important part for now.