No, that's qualified nonsense. No one has ever tried that because 1) it wouldn't work and 2) it be a huge waste of resources to even try.

Now the second problem is that terraforming is a modern word more applicable to science fiction than historical reality. The closest to this we can come in a mediaeval setting is the slow process of clearing new land: cutting down forests, pulling stumps, removing rocks, draining watercourses and swamps creating the landscape of Europe of today. Which is important to note is effectively entirely man-made. Which means that sure basically all of Europe has been terraformed, but in a process covering at least a millennium. This is not something you effect within the context of a battle or siege.

Now if we limit the idea more concisely to using the terrain and modifying it to suit a conflict, then yes it happened. And yes there were specialised tools for it, namely: spades, hoes, picks, shovels, axes and other such implements. And a boatload of human work, maybe assisted with animal power. Taking advantage of terrain or modifying it to suit your needs was fairly common. The peasants of Dithmarschen in a battle in 1500 opened up sluices flooding the areas making it difficult for the larger invading forces to effectively work, and the peasants better acquainted with their terrain managed to soundly defeat the invaders (said peasants had spent centuries jealousy guarding their independence from outside authorities in their marshy and impassable homeland). Similar places like the Dutch lowlands also used this on many occasions. In forested areas with few paths inferior forces would barricade the paths with fallen trees and ambush their opponents, a tactic Swedish peasants often used against their overlords, be they Danish medieval kings or the Reformation era developing early-modern Swedish monarchy. These examples however do rely on existing features, the walling and diking of marshy lowlands was a matter of flood control and land reclamation, and obviously a forest grows not in response to your need to create a nice place for ambushes. So to some degree it was influenced by said general terraforming efforts it was really more that convenient features happened to exist less than an active method created specifically for an event, say a battle.

Field works and fortifications were used if applicable, most commonly in conjunction to sieges, where trenches and earthen ramparts and walls and palisades were often built. Naturally mining efforts would also fall under this label. It's not all about sieges and gunpowder though, the Swiss used earthwalls and palisades to great effect at Näfels 1388 and Voegelisnegg 1403 and Stoss 1405, in the latter two cases as a deliberate trap to lure the enemy to break through it and be trapped behind it. (And these are just from a quick skim of Osprey Men at Arms 094 "The Swiss at War 1300-1500" while I was looking for a specific battle that happened maybe just later than mediaeval proper. As the mediaeval period wore on and firearms of various kinds became more widespread and powerful throwing up some earth in front of you became ever more useful. The most comprehensive "terraforming" efforts would be associated with the building of castles. The Norman motte and bailey style e.g. could mean quite the extensive construction of a hill if one didn't naturally exist and the extension or creation of water features. Constructing castles would greatly impact the local environment, but again this should be more considered a permanent change of lands for overarching reasons not something you do for one situation, and certainly not in the sense that it was made with siege weapons. I've mostly spoken here about Europe here but naturally other places also change the world around them. I recall some examples of Japan of extensive engineering efforts in relation to sieges both in besieging and in building castles but I'd have to really read up first to say. I vaguely recall Oda Nobunaga rerouting a river or creating a lake or something like that in his protracted conflict with the Ikko-ikki, but it could be argued falls outside the medieaval period.

For an overview of how mediaeval siege equipment was actually used I'd suggest e.g. Osprey Essential Histories "Medieaval Siege Warfare" and New Vanguard 043 and 044 "Siege Weapons of the Far East 621-1300 and 960-1644" Volumes 1 & 2 respectively.

In short, terraforming is not exactly the concept I would apply to using existing terrain features, adapting/modifying them for defensive or offensive purposes. Military engineering in general was broadly used when opportunity arose to create from scratch defences if natural terrain wasn't amenable. However, in none of these cases would you use a siege weapon to do that. You used the tools of the builder and engineer.