I'm having a real problem with tense...

I thought the same thing when I read it. That was the other problem I had with the sentence in addition to the comma splice. Just say "slot" and leave it at that.

 

Depends, doesn't it? We can't judge from one sentence whether "purpose-built" is necessary/important information or not.

 

Given the choice between past and present tense, I much, much prefer to read past. To me, reading something in present tense is janky and unwieldy. Present tense makes me feel like I'm standing in the middle of the room observing each character and what they're doing, and making notes in my head:

"She walks across the floorboards, each creak as loud as a thunderstorm, and reaches for the knob that leads to the Earl's study."

In contrast, past tense makes me feel like I'm a reader; like I'm following along just behind the story, and being lead through each event as it happens:

"She walked across the floorboards, each creak as loud as a thunderstorm, and reached for the knob that lead to the Earl's study."