Ah I see, I thought that building had a pointy top but when thinking about it it probably doesn't; you enter the level through a hole in its roof, right?

Yes.

I may be wrong but aren't there gems on the backside of that ledge on said building?
So technically "getting on top of it" isn't a particularly new achievement

Nice to find your way to its front though!

You're getting confused. The gems are behind the building. In this picture, Spyro is on top of the building.

Nice to find your way to its front though!

I have managed to get on top of the first building in Cliff Town.

Here it is:

I got there by going to the object with the spikes, jumping, then quickly tapping Square then X. Tapping X after tapping Square makes it possible to stand on the spike without headbutting the object. Then, jump up onto the the next spike, and jump and glide to the building, and charge just before you reach the building to clear the gap. If you are playing the NTSC version, then you will probably have to do a charge-glide by tapping Square just before you press X. Charge-gliding gives you a bit of extra distance to your glide, and charge-gliding here will give NTSC players compensation for the extra jump height that the PAL version has.